A Sound plays a specific AssetSound when the player enters a rectangle defined on the XZ plane, which is used to "trigger" the sound. One can also specify whether the sound should loop once triggered, or only play back once. To get ambient sound or music to play for the room upon entry, use a very large rectangle to trigger the sound (or at least contains the room's entrance portal), and set the sound to loop.
http://janusvr.com/docs/build/roomtag/index.html#Sound
loop (default "false") - normally the sound plays only one time, but when this attribute is set to true, the sound will play indefinitely until the player leaves the room
play_once (default "false") - when set to true, the given Sound will only play one time for the duration of the visit. If set to "false" (the default), the sound will play once each time the player enters the room.
pos (default "0 0 0") - when set to any other value than the default, the Sound is 3D spatialized (requires version 45+)
dist (default "1.0") - distance attenuation setting for 3D spatialized sound (requires version 45+)
gain (default "1.0") - gain setting for 3D spatialized sound (requires version 45+)
pitch (default "1.0") - pitch setting for 3D spatialized sound (requires version 45+)