May have just found a simple work around: Created a blank text document in TextEdit (Mac) and then renamed it with .sf2 extension in Finder. Then in SeeMusic, Advanced, clicked on MIDI Sound file name, Imported the new blank soundfont and Confirmed as selection. I had looked into creating a blank .sf2 with PolyPhone, but there is apparently an issue with it and OS X (Catalina)
That is a solution that could definitely work.
The normal solution at this time would be to turn off the MIDI Synth in the Player window - see attached screenshot:
Does that help?
Thanks for using SeeMusic!