I was able to reproduce this : I had one hum.wav and one humM1.wav in the same folder (WIP font). The font lookup routines counted both as hum files and the heuristics decided polyphonic font based on the presence of humM1 but at the end it considered 2 available hum
As a result, it tried to pick sometimes humM1.wav and sometimes (randomly) humM2.wav since the board supports multi hums in polyphonic mode, but obviously, humM2 didn't exist => silence.
Logging this over the serial port would tell you so. I just removed the hum.wav from the font and it works normally. The 2 files shouldn't be in the same directory anyways since it could confuse poly vs mono mode, the naming is used to differentiate.
(obviously, I can always strengthen this in the code)