Fanyo,
I think your issue may be a different than Tyler. If you are experiencing a “dip” in volume as you move the treadle, that is caused by the volume level table in the microcontroller not being populated correctly. If you run the stereo setup routine, that will re-populate the table and is likely to fix it.
To directly answer your questions, my suggestion about adding a capacitor was only theoretical. I have never tried it myself. It will take some experimentation to find the exact capacitor because I can’t remember the exact frequency of the PWM, but I think it’s around 30kHz.
-Brach