Here is what you could do:

If you create an adventure map, you probably will create resources for it anyway...

You could split the file into multiple short ones, and make a repeater row that keeps playing them one after another, then simply check when the player leaves the area and stop the circuit and start another.

A quite complicated way and not lagfree either, but I guess it could be the best one right now, also I'm not sure how smooth the file switching would work...

Maybe you want to try that!

~tobsam

It is currently impossible to do this, there is no option to stop current /playsound's. Unless you set coordinates for it to play, and the players slowly back away from it, then it'll stop.

I actually toyed with some concepts like this for my server, so that I could use it to try and put theme songs into each nation's area, and the best you'll get is a carefully engineered radius setup like atomick suggested, so that they exit the radius of one as they enter another. doing it only really becomes practical if it's used on wide area scales, however, due to the nature of command blocks.

I recommend that if you're looking to use it for a town type area with a lot of those transitions, you just give the town it's own theme, and other stuff it's theme, rather than an inside and outside transition on all of them, which would lead to a LOT of lag, repetition on your part, and likely get irritating if they're going in and out a lot.

You might be able to find the perfect distance so that when they are out of the building, they can hear it easily, but inside they are too far away to hear it

Not sure this is possible. I am pretty sure music is client side. If they have a certain specialized resource pack without the music this can be achieved though because the music won't play.
EDIT: (if you delete the music files from resource pack)

Can you tell me how?

I can pm me the ip and ill do it or do i