ambientmusic.2da is all that is needed to get Area music working.
For sounds and placed sounds in the area - you just need to keep the sounds as a wav file type, and then have them present in the override directory.
Note - I use alot of custom music and sounds in my PW.
I discovered that wav's, and BMU's are considered 'optional' extras to nwn.
Meaning, if you keep the content, outside of a hak file, then the players of persistent worlds, can still get into that pw, even if they dont have the content. They wont hear the content, but they wont be booted either.