Soundscape is one of the most neglected aspects of environment design in SL. Partly due to platform and scripting limitations and partly because anecdotally many people seem to mute them or to play music and so miss them that way. The main barriers are the falloff distance being very sharp for some sounds - people can't hear them unless they are up close or the volume is set very loud, a persistent bug that prevents sounds from reliable playing for all listeners and the difficulty of making longer loops with enough variation not to sound odd or annoying. Sound becomes a secondary aspect to the design, much like lighting, when your expertise lies elsewhere and you are pretty sure many of your audience will not experience it at all or will experience it badly.
This sounding on sound sounds right. (See what I did there?) Overwhelmingly, the soundscape of Second Life is streaming music and voice chat, neither of which contribute much to immersion (and in fact, tends to mar it).