I am experiencing the following issue with MIDI playback (I am using a recent version of Fedora, and improvisor 6.0)
The first time I started improvisor, I selected Gervill for MIDI output, and everything worked fine (although the sound was particularly hi-fi).
Then I installed fluidsynth on my computer (including the corresponding soundfont files /usr/share/soundfonts/default.sf2 and /usr/share/soundfonts/FluidR3_GM.sf2) and restarted improvisor. Apparently, Gervill started using the new soundfonts -- the sound was generally better. The problem, however, is that the percussion was just wrong. It seems that the snare drum or bass drum was replaced by a shaker.
On the other hand, if I export the leadsheet to MIDI and play the MIDI file through fluidsynth, the strange shaker sound is not there.
I also noticed that this problem doesn't happen in version 5.16.
I've got the same problem using Ubuntu 14.04.1 - changing soundfont for Gervill causes wrong sound of percussion.