I use a software synthetizer ( zynaddsubfx) and in this way rosegarden
playes.
You have to start zynaddsubfx ( with dynebolic I don't have to start jack,
just zynaddsubfx and then rosegarden).
"It's easy to map the instruments to midi channels:
zynaddsubfx has 16 parts and you can map each channel to any number of
parts. In the main window, search for part control and change the current
part control. Each part has a "Midi Channel Receive" control, where you can
choose to which midi channel to respond (by default the part N is mapped to
midi channel N). If you changed the right part, you can load the instrument
to the desired part.
You can save all zynaddsubfx settings (currently loaded instruments, effect)
using the file menu."

Stefano