you can't
the audio from all the programs is summed by to the Windows audio subsystem, if the individual programs don't offer volume controls then you're fucked
( Gilgamesh gazed in wonder as Frampton came alive, Sat 6 Jun 2009, 15:40, archived)

Thanks
Guess I'm fucked then :D
( TIAL Four capital letters, written in gold., Sat 6 Jun 2009, 15:42, archived)