USB-audio feedback (USB AudioStreaming Isochronous Feedback) is indispensable for stable sound playback without clicking noise. I added simple feedback function by checking the vacancy of the buffer. To avoid modifying other sources (such as audio.cpp in pico-extras), number of free buffers is checked in this source code.
USB audio also has mute function (mute without changing volume value). I also added this function (in the original source code, it is almost done, so I added just three lines).