[crunch]

The game has six channels for playing background music. aux1, aux2......up to aux6.
aux1 will play throughout the entire level in all areas when used alone.
When used in conjunction with the other channels, it is designated as "outdoor" and needs to be triggered as such.
The same holds true for aux3 and aux5. They need triggers of "outdoor".
aux2 aux4 and aux6 are designated as "indoor".

To work this-- make a trigger_multiple just outside your door, and a trigger_multiple just inside your door. These triggers will set the correct channel for your music. Each trigger needs a $targetname/interior or $targetname/exterior, along with a #set.

The #sets are as follows:
For triggers of $targetname/exterior:
#set/1 will be aux1
#set/2 will be aux3
#set/3 will be aux5

For triggers of $targetname/interior:
#set/1 is aux2
#set/2 is aux4
#set/3 is aux6

In your .mus file you will "point" to these aux channels like so (this is an example, your names will be different.)

path sound/amb_stereo

normal Amb_road_ext.mp3 // Plays over entire level
aux1 Amb_road_ext.mp3 // Exterior 1 - all exteriors
aux6 Amb_church.mp3 // Interior 1 - all interiors
aux4 Amb_road_int.mp3

!normal volume 2
!normal loop

!aux1 volume 2.0
!aux1 loop

!aux6 volume 1.5
!aux6 loop

!aux4 volume 1.0
!aux4 loop

I have named my .mp3 files "Amb_mymap_int" etc...and placed them in my main-->sound-->amb stereo (NOT amb! MUST be "amb stereo")

As you can see, my music file points to my file and plays when triggered.

I do not know how or even if an individual sound can be "attached" to a player. Just that you can get different music in different areas of your map.
Hope this little novel I just wrote helps!