Installing Cubase on more than one machine isn't a problem so long as you don't mind switchin the dongle (if you have one) everytime you switch computers.
As for the headphones - i'm assuming you've turned the 'PHONES' up...
If so, do you get playback when listening to Media Player or whatever player you use? If you do, and it's just within Cubase that you get issues, then try looking at VST connections in the Cubase upper menu.
If you don't get playback at all, then go to your control panel, go to your sound device properties, make sure all the right things are selected. The tascam should be selected as your soundcard for all purposes.
If it's just FL9 check the options or sound properties of that program. Is there sound coming from the Line Out? If you got speakers attached, is there sound coming from them?
I don't know too much about smaller interfaces unfortunately, but i saw that nobody had posted so I thought i'd at least give you a few thoughts!
Hopefully you get it sorted.
