Situation:  Voldemort is getting his Death Eaters ready to wage war.

Song:  “Be Prepared” from The Lion King

Voldemort (to the Death Eaters):
I never thought dementors essential
They're crude and unspeakably plain
But maybe they've a glimmer of potential
If allied to my vision and brain

I know that your powers of retention
Are as wet as a warthog's backside
But thick as you are, pay attention
My words are a matter of pride

It's clear from your vacant expressions
The lights are not all on upstairs
But we're talking power and successions
Even you can't be caught unawares

So prepare for a chance of a lifetime
Be prepared for sensational news
A shining new era
Is tiptoeing nearer

Death Eaters:
And where do we feature?

Voldemort:
Just listen to teacher
I know it sounds sordid
But you'll be rewarded
When at last I am given my dues
And injustice deliciously squared
Be prepared!

Wormtail:
Be prepared for what?

Voldemort:
For the death of Dumbledore!

Wormtail:
Why? Is he sick?

Voldemort:
No, you fool, we're going to kill him...and Harry too!

Wormtail:
Great idea! Who needs Hogwarts?

Death Eaters:
No Dum-ble-dore!  Nah nah nah nah nah nah!

Voldemort:
Idiots! There will BE a Hogwarts!

Wormtail:
But you just said, uh...

Voldemort:
*I* will run it!
Stick with me, and you'll never be in Azkaban again!

Wormtail:
YAY!!!!!! Long live the Headmaster!

Death Eaters:
It's great that we'll soon be connected
With a guy who'll be all-time adored

Voldemort:
Of course, quid pro quo, you're expected
To take certain duties on board
The future is littered with prizes
And though I'm the main addressee
The point that I must emphasize is
You won't get a sniff without me!

So prepare for the coup of the century

Death Eaters: (Oooh!)

Be prepared for the murkiest scam

Death Eaters: (Oooh... La! La! La!)

Meticulous planning

Death Eaters: (We'll have power!)

Tenacity spanning

Death Eaters: (Lots of power)

Decades of denial

Death Eaters: (We repeat)

Is simply why I'll

Death Eaters: (Endless power)

Be there undisputed

Death Eaters: (Aaaaaaah...)

Respected, saluted

Death Eaters: (...aaaaaaah...)

And seen for the wonder I am

Death Eaters: (...aaaaaaah!)

Yes, my wand and ambitions are bared

Death Eaters: (Oo-oo-oo-oo-oo-oo-oo)

Be prepared!

Death Eaters:
Yes, our wands and ambitions are bared

Voldemort & Death Eaters:
Be prepared!!!!!!!

Book: Goblet of Fire
Situation: Harry has just found out about the Avada Kedavra curse that Voldemort used to kill his parents.
Song: "Hakuna Matata" from Lion King.

Ron:
Avada Kedavra
What a horrible phrase

Harry:
Avada Kedavra
You'll see a big green blaze

Harry & Ron:
Before you realize
That's the end of your days
It's Voldy's moral-free
Malignity
Avada Kedavra

Ron:
(speaks) Yeah, take Harry for example.
(sings) Before he was at Hogwarts

Harry:
(opera style) Before I was at Hogwaaaaarts!

Ron:
(digging in ear with pained expression on face) Lovely, Harry

Harry:
(spoken) Thank you.

Ron:
His parents were nice, young Lily and James
A great witch and wizard with well-known names

Harry:
Then Voldemort came - through our door he burst
And he killed them, with that one atrocious curse
And oh!  The pain!

Ron:
Yes, he was in pain

Harry:
Thought of changing my name!

Ron:
Oh, what's in a name?

Harry:
'Till I went to Hogwarts

Ron:
Where did you go?

Harry:
It's located in--

Ron:
(covering Harry's mouth) Harry! Not in front of the Muggles!

Harry:
Oh. Sorry!

Harry & Ron:
Avada Kedavra
What a horrible phrase
Avada Kedavra
You'll see a big green blaze
Before you realize
That's the end of your days
It's Voldy's moral-free
Malignity
Avada Kedavra

Book: None, actually. This is in the Marauder-Era.
Situation: The Marauders being who they are make this their theme song!
Song: "Prowlin'" from Grease 2

Moony:
Well, come on and tell us, Padfoot

Prongs:
What's the secret of success?

Padfoot:
You gotta take a tip from the kiiiing of hip
Cuz you knoooow that heeee's the best

All:
We're goin' prowlin'
We're goin' prowlin' tonight!

Padfoot:
I see you're hungry for a lover,
Gotta find a chick who'll give you more.
Well, there's a spot that I've discovered
Where a guy's guaranteed to score.

I'm gonna show you cats some action,
Like you've never seen before.
We're gonna get some satisfaction
Gorgeous girls galore!

Padfoot:
We're goin' prowlin'

Moony, Wormtail, & Prongs:
Walk, talk like Marauders

Padfoot:
Prowlin'

Moony, Wormtail, & Prongs:
Walk, talk like Marauders

Padfoot:
Prowowowlin'

All:
Tonight!
 
Prongs (spoken):
Well, I like a debutante, who comes across
Now that's what I call class.

Wormtail (spoken):
Well, I like a tall girl, with long legs that go right up to her—

Moony (spoken):
—You know what I like?
I like a girl that's really smart
Provided that she's really stacked

Padfoot (spoken):
Yeah? Well I love 'em all, and they looooove me.
Cuz I'm the leader of this here pack!
We're goin' prowlin'

Moony, Wormtail, & Prongs:
Walk, talk like Marauders

Padfoot:
Prowlin'

Moony, Wormtail, & Prongs:
Walk, talk like Marauders

Padfoot:
Prowowowlin'

All:
Tonight!

Padfoot:
We're goin prowlin'

Moony, Wormtail, & Prongs:
Prowlin'

Padfoot:
Prowlin'

Moony, Wormtail, & Prongs:
Prowlin'

All:
Toniiiiiiight. Tonight!

Book:  Probably Prisoner of Azkaban, or any book after
Situation:  I have Moony singing this in my fanfiction, so I thought I would put it up here because I think it's hilarious.
Song:  "Werewolves of London" by Warren Zevon

Remus:
I saw a werewolf with a Chinese menu in his hand
Walking through the streets of Soho in the rain
He was looking for a place called Lee Ho Fook's
Going to get himself a big dish of beef chow mein

Chorus:

Ahh-ooh, werewolves of London
Ahh-ooh
Ahh-ooh, werewolves of London
Ahh-ooh

If you hear 'em howling around your kitchen door
You better not let 'em in
Little old lady got mutilated late last night
Werewolves of London again

[Chorus]

He's the hairy-handed gent who ran amuck in Kent
Lately he's been overheard in Mayfair
Better stay away from him
He'll rip your lungs out, Jim
Heh, I'd like to meet his tailor

[Chorus]

Well, I saw Lon Chaney walking with the Queen
Doing the werewolves of London
I saw Lon Chaney, Jr. walking with the Queen
Doing the werewolves of London
I saw a werewolf drinking a piña colada at Trader Vic's
His hair was perfect

Ahh-ooh, werewolves of London
Ahh-ooh
Ahh-ooh, werewolves of London
Ahh-ooh
Draw blood
Ahh-ooh

Book: Order of the Phoenix
Situation: The Order's theme song about the coming war with Voldemort.
Song: "Invincible" by Pat Benatar

The Order:
This bloody road remains a mystery
This sudden darkness fills the air
What are we waiting for?
Won't anybody help us?
What are we waiting for?

We can't afford to be innocent
Stand up and face the enemy
It's a do or die situation
We will be invincible

This shattered dream you cannot justify
We're gonna scream until we're satisified
What are we running for?
We've got the right to be angry
What are we running for?
When there's no where we can run to anymore

We can't afford to be innocent
Stand up and face the enemy
It's a do or die situation
We will be invincible
And with the power of conviction
There is no sacrifice
It's a do or die situation
We will be invincible

Won't anybody help us?
What are we running for?
When there's no where, no where we can run to anymore

We can't afford to be innocent
Stand up and face the enemy
It's a do or die situation
We will be invincible
And with the power of conviction
There is no sacrifice
It's a do or die situation
We will be invincible

Book: Prisoner of Azkaban
Situation: Since the first time I heard this song, I always pictured Sirius singing it from Azkaban, even tho the song is getting rather old now. 
Song: "Bring Me To Life" by Evanescence

Sirius:
How can you see into my eyes like open doors?
Leading you down into my core
Where I’ve become so numb
Without a soul my spirit sleeping somewhere cold
Until you find it there and lead it back home

Chorus:

Wake me up inside
Wake me up inside
Call my name and save me from the dark
Bid my blood to run
Before I come undone
Save me from the nothing I’ve become

Now that I know what I’m without
You can't just leave me
Breathe into me and make me real
Bring me to life

[Chorus]