You could use 2 VCAs in series - the first with an envelope generator attached to it's CV in and the second with a CV velocity signal going into its CV in (thekenton pro2000 midi-cv convertor for example allows you to output velocity as CV from one of its 6 aux outs). Then just pass the audio through VCA 1 audio in, to VCA out, then into VCA 2 audio in, to VCA out into your mixer or whatever.
-Chaz