Instruments are out of tune or sounding otherwise unexpectedly odd.
You most likely have an external keyboard or other MIDI controller connected that happens to send pitch wheel or modulation wheel data, which is forwarded by MIDI Through to the currently selected instrument in Synfire. If you switch tracks, other instruments will get out of tune or modulated too.
Disable MIDI Through in Synfire or disable these controllers with your external keyboard, if they don't reliably reset to zero or neutral state when released.
If a particular sound is very sensitive to external controllers, you might also enable the option "Reset Controllers On Sound Selection" for its device.