Tremulous sound files that are used as sound effects in maps should be 22050 KHz 16 bit mono. They must be uncompressed and stored as PCM inside of WAV containers. Simply said: They must be uncompressed WAV files. Sadly, MP3, OGG etc. are not supported by any Tremulous client.
It's obvious that the sounds have to be mono because otherwise, how would they be made to appear to come from the left or the right if they were already defined how to sound like for the left and the right ear? This begs the question whether or not sounds that are defined to be played globally could be stereo. Maybe someone can test this with an old client.
If you really want to have a stereo impression somewhere in your map, you have to place two different mono sounds in two places.
Music WAVs that are defined in the worldspawn can be stereo.
Sounds with other parameters (for example slower sample rate) have been seen in the Tremulous world (for example the well known and partly incorrect lecture about the word "fuck" that is used as an Easter Egg in the map "Highrise"). But this has to be exploited very cautiously. If you would do so, then you should design the map on an old client (like the original Tremulous 1.1.0 client or the TJW backport) to make sure that it will probably work everywhere. You should develop maps on those old clients anyway. They do for example sometimes have problems with incorrect map files, but these problems do not occur on newer clients like TremFusion. Since many players still use older clients, it should be made sure that they won't have problems, so an old client should be used for testing/developing maps.
3D sound sources are point sources, and it cannot be defined how big their center is, how fast they fall off in the distance or anything else. If you want to create the impression that the loud center of a 3D sound source is not a point but a space in which you can move without experiencing a change in sound, you have to place several identical sound sources in a relatively close proximity. This might also cause the sound to be audible from a greater distance.