The song is from season 4 episode 14 of MLP:FiM.
Main song:    • MLP:FiM | Music | Find the Music in Y...   | ♫ Lyrics for the song:
( You can support the show here: https://itunes.apple.com/us/tv-season... )
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[Big McIntosh]

[doo-wop-style scatting]

[The Ponytones]

[harmonizing]

[Big McIntosh]

Everypony's sayin' you should learn to express your voice
But if talk doesn't seem like it's the answer

[The Ponytones]

Luckily you have a choice
When you find you've got the music
Got the music in you
Find you've got the music
Got the music in you

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[Fluttershy (male voice)]

[doo-wop-style scatting]

[The Ponytones]

[harmonizing]

[Fluttershy (male voice)]

Trot outside and you see the sunshine
Something's in the air today
Sky is clear and you're feelin' so fine
Everything's gonna be a-okay

[The Ponytones]

If you listen carefully
On every corner there's a rhythm playing
Then it happens suddenly
The music takes you over and you'll

Find you've got the music
Got the music in you
Find you've got the music
Got the music in you

[Fluttershy (male voice)]

Oh, o-o-oh
Everypony's sayin' you should learn to express your voice
But if talk doesn't seem like it's the answer

[The Ponytones]

Luckily you have a choice
When you find you've got the music
Got the music in you
Find you've got the music
Got the music in you

Got the music, got the music in you!

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[The Ponytones]

[harmonizing]

[Fluttershy (male voice)]

Trot outside and see the sunshine
Something's in the air today
Sky is clear and you're feelin' so fine
Everything's gonna be a-okay

[The Ponytones]

[harmonizing]

Fluttershy: [deep male voice] We wouldn't want to disappoint the spa patrons.

[The Ponytones]

If you listen carefully
On every corner there's a rhythm playing
Then it happens suddenly
The music takes you over and you'll

Fluttershy: [deep male voice] We wouldn't want to disappoint the schoolponies.

[The Ponytones]

Find you've got the music
Find it in your heart, you can
Find it, oh yeah
Got the music in you now

Got the music, got the music in you!

---

[Fluttershy (male voice)]

Hey, find it, oh why, you can find it
In your heart, yeah, find the music, yeah
It's in you right now! Oh, yeah!
You can find the music!

[crash, music fades]

Fluttershy (male voice)]

Yeah, yeah, I love the music, ye—

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[Fluttershy]

There's music in the treetops
And there's music in the vale
And all around the music fills the sky
There's music by the river
And there's music in the grass
And the music makes your heart soar in reply

[The Ponytones]

When you find you've got the music

[Fluttershy]

You've got to look inside and find

[The Ponytones]

Find you've got the music

[Fluttershy]

The music deep inside you

[The Ponytones]

Find you've got the music

[Fluttershy]

'Cause when you look inside, you'll see it

[The Ponytones]

Find you've got the music

[Fluttershy]

You're gonna find, you're gonna

[The Ponytones]

Find you've got the music
Got the music, got the music
Got the music in you!

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"My Little Pony" is owned by Hasbro,
"MLP: Friendship is Magic" is created by Lauren Faust.