I sent this reply to the email you sent support, but I'll add it here too:

If your MIDI playback is using the Microsoft GS Wavetable Synth, that will not be rendered into your MP3 file as the GS counts as an "external" synthesizer. Please go to Options > Preferences > MIDI Driver, then set it to use a VSTi/DXi synth - the included CoyoteWT synth should sound the same as the Microsoft GS.

[Linked Image - Only viewable when logged in]


I work here