[LAU] Problem getting started with Midi editing - no sound!

I have hesitated for weeks to post here with such a basic and possibly stupid problem, but I am at my wits end with this. I run Ubuntu 12.04 on a desktop computer and also on a small notebook. Sound is fine on both of them, and I can play midi files through tiMidity with no problem. I want to use a midi-capable electric piano as an input device, and I reckoned the first thing to do was to get some suitable software and make sure I could play a midi file from a midi editor. I have QjackCtl, Qtractor (I also have Muse) and Qsynth installed. I have spent weeks, on and off, trying to solve the basic problem of getting some sound from Qtractor (or Muse), but although both the editor and QjackCtl show that an imported midi file is actually being played, I get no sound whatsoever. This is true for both computers, even though their hardware is pretty different. As I said earlier, the sound on both computers is fine in other contexts, so it is almost certainly some fundamental error on my part, but I have no idea what is wrong. I have tried all sorts of different sets of connections in QjackCtl, but never heard a note yet! Having read various posts in the forum archive, I tried running jack-metro. QjackCtl shows that the metronome is running, but connecting the metronome to the system outputs produces no sound, so the problem seems to be at this level. If anyone feels they have the patience to try to point me in the right direction, I shall be very grateful indeed. David