I understand that you have had numerous requests for this and you always go back to AGC. However, this missing feature is the only reason why my guild of 40+ will not move over to it. We tried it for a few weeks and they hated not being able to change individual volume control. Some people were too soft, some way too loud. Trying to explain to A LOT of people how to get their sound level correct via audio wizard and amplification is tedious and many people would just rather go back to what they know.
I understand that everyone should not have to suffer because of a few people, but it really is a hassle trying to tell someone how to fix it step by step. I do my best to do so but people would just rather change someones volume instead of directing the how to do so. People are used to Ventrilo and are afraid of change, why not put this option in (it won't HURT anything) and help some people move over to mumble more easily.
I know I am not alone in this camp either, almost everyone wants it. Who doesn't want more options/customization? Anyways, I just thought I'd voice that this is the only reason we do not use mumble anymore (even though I love all it's other features).