I just opened the case as described above, took the screw out holding the board in, gently pried the volume wheel off so I could get to the sealed potentiometer (which is the volume variable resistor), and lightly blew on it with little effort. I was very surprised to see it go from crackly and almost impossible to listen to, to like new.
Mine stopped working altogether. No sign of life even with the power plugged in. I had tried the "reset" steps I found on the internet (holding big button down then plugging into power) and also a USB cable still with no lights or signs of life. I disassembled it, unplugged the battery, plugged it into power without the battery and it worked. Then I unplugged the power, plugged the battery back in, and plugged it back into power. It fully charged in a couple of hours and the battery measured 12.58v (with no load) after it was fully charged. It works fine now. I think the battery is probably three 18650's in series with a protection circuit.