I too had been suffering with bad VOX quality until yesterday. I spoke with Skuzzy and he explained that the VOX system looks at the overall performance of the game, plus your connection in order to determine the best VOX setup (I assume this means codec and sampling rate).
I know that I have my video settings and graphics set at the point to get the best possible picture while maintaining a good frame rate (normally 59 but would vary going onto a furball or over strat city). So I dropped back to 512 textures and down to x2 AA, guess what VOX now sounds great! I am going to try a couple different video settings to see if I can get back to the Hi Res textures, but at least I know how VOX works and can make more adjustments.
So if you are experiencing bad VOX quality in game try lowering your video and graphic settings just a bit and see if the VOX gets better.