Disklavier

A have an old disklavier that plays only 720k floppy.I attached a 
laptop and it works fine. My problem is that I cannot adjust the 
volume while the piano is playing. Im using Vanbasco and Cakewalk.
I guess this is the disadvantage using a laptop rather than playing 
it with the regular diskette w/c can be adjusted as I please. Thanks 
for any help.
Allan