Also Hey Fun Fact About Engineers Theme: Its Actually Ripped From The Song Someone Else's Song By Wilco - Tumblr Posts
Engineer's theme song is one of the few songs WITHOUT any backup. Obviously right? Because Engineer is the one playing the song.

Out of all the 33 tracks, just Engineer's theme has no backup. Only his, and when there is backup, it's an bass guitar.
But, here's the thing, all the songs are jazz, with the added instruments to go with the characters (Scout has bongos, Demo's bagpipes, Soldier's trumpet, etc)
But Sniper's? His is only saxophone. The saxophone is present in EVERY single song, in EVERY single one of the tracks. Including Engineer's.
MEANING, THIS MOTHERFUCKER MADE ALL THE SONGS:

I actually lied about everything I just said.
But now that you're here, lemme tell you about what all the instruments each merc plays (FYI, I won't be mentioning taunts used twice, like the guitar, cymbals and drums. I'm only using ones unique to their characters):
Obviously Sniper and Engineer play the guitar and saxophone, as you can see. I think Sniper was also playing the saxophone in Expiration Date? But don't quote me on that.
Soldier plays the trombone, which is funny considering his theme songs are usually accompanies by trumpets. But the more you know.

Pyro has a taunt where they use their axe like a guitar, so we can assume they can play the guitar too.

Scout plays the bongos. Since almost everyone on the TF2 team can play an instrument, he for sure was playing the bongos in his own theme song:

Medic has a taunt where he plays his bonesaw like a violin, so we can assume he plays the violin.

Heavy doesn't play any instruments. I'm somewhat disappointed he doesn't have a taunt where he plays Sasha like a cello, since he would've matched with Medic's taunt. But it also makes sense with his backstory and personality, he's more of a listener then anything else.
Spy doesn't play an instrument either, but something I heard was that Spy's voice actor copies Scout's voice to make Meet The Spy more authentic, since Spy's disguise (nudge nudge wink wink to anyone who gets that reference) doesn't mimic the mercs voices. Though I'm not too sure if that's true, so don't quote me. I'm guessing Spy has a sickass singing voice considering he can voice act really well.
And finally, Demo. He plays the piano and bagpipes as seen in his taunts and probably Expiration Date. Again, for the third time, don't quote me because I couldn't find the original scene this was in.

