just spitballing here.....for pure XML modlets...and not counting who owns the music:
Maybe replace the "horn" with music? You'd only get 1 song, likely.
Does anyone know how to detect (like a cvar or something) if a player is "in or driving" a vehicle? If so it might make sense to be able to trigger a buff and play a sound (music) in the players head... if only 1 person could drive ( or can detect "you're driving") then can just play that sound on that player and not in the players head. The find some way to handle triggering the buff based on an item the player has...in their hand/inventory?
Alternatively, you could possibly (and this is terrible) replace the "highest gear" driving sounds of the vehicle with the music....which would mean you craft it with like, a single 8 track in it forever... which is possible in real like if you've ever owned an 8-track (may you rest in peace, my first car). Alternatively add recipes to "craft" a new 4x4 with a r4x and a new music item. Its a terrible solution ... and if you slow down the music stops.. .but then again they made that movie "Speed" and it was a hit so in a way its not a totally inconceivable concept ;0
Other than that, maybe when vehicle mods come in it will be do-able? Another possibility: xyth? made "music player" blocks. I've heard that TFP doesn't plan on adding places blocks can be placed to vehicles ... but in some of the "boats and rafts" discussion there has been talk of maybe allowing a block or 2 to be placed on/on a raft.
All other options that come to mind are maybe literally non-vanilla vehicles doing some deeper level of non XMl magic magic to allow this just for them.