I'm a little rusty myself but I did write just such a script years ago for my InfraRexx setup... How about something like this:
ADDRESS AMPLIFIER.1
'VOLUME'
MyVol = RESULT
MyVol = MyVol + 10
IF MyVol > 64 THEN MyVol = 64
'VOLUME 'MyVol
Think that should work to up the volume, and check to make sure you don't go above the maximum value of 64. Bear in mind that different players have different volume scales (IIRC AmigaAmp goes from 0 to 26), and that I remember steps of 1 were far too small to be useful or even noticeable, so I think I used 5 or 10. Experiment there...