It's a common made error with DJ's in SL
Music is to loud and when the talk you not hear it because it's to soft.
Sansar have sort of same problem. voice volume is not on the correct volume, a bit soft i need to push my fasder above the 0 level on meetings. But on experiences many times the sound volume is way to high. And you cannot correct it at this moment. And this is going to be hard to get correct from the experience side. because every audio system and every sound card is different.
So a client side option to adjust the volume for the voice channel and the sound channel separate is really a must have. Let users always have the control on volume. Every game i know have volume options in settings.