MOTM

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