problems playing midi files; no sound heard

[Amos Tibaldi]

Hello,
  I am using Ubuntu Studio 9.04 and jack is working correctly. I can
hear a sound i.e. when I
start Audacity that connects to jack and I play a generated tone, but
when I use muse and import a midi
file I cannot hear any sound even if muse seems to proceed playing.
The same happens with lmms.

What can I do?
Thanks in advance

AmosT