Although I haven't tried it yet ... I'd imagine you could use something like:
setVolume(int level)
Set the current volume of the sound between 0 (off) and 100 (loudest.)
and if you wanted to do it for individual sounds you could do something like:
Greenfootsound.setVolume(High or Low int)
*play sound here*
Greenfootsound.setVolume(back to original volume level)
Most of this is in the GreenfootSound class documantation.