Been there done that. Its common mute except ESU
Still head scratching. Tried resetting to factory as well.
F8 is a prime mover mute function for the sound projects of U.S. locomotives (when the loco is in motion). When loco is stopped then F8 performs the prime mover shutdown/startup. So it has a dual functionality.
To mute all the sounds any time they usually use F31. It is all easily remapable if one so desires.
Also good to know that you were able to find someone local to you with a LokProgrammer.
But that's not what we are supposed to be discuss here.

As for the problem, your guess is as good as mine. I believe that Bachmann sound decoders are made by Soundtraxx. It could be the speaker, broken wire, or some faulty part on the decoder. You could contact Bachmann if the model is still under warranty.