It is not an original Pi power supply but as I had it working correctly, I did not thought about it at first but as you asked I measured again the voltage. I suspect the supply somehow degraded. I read about 4.95V in input. I should definitely find an official 5.1V supply to try.
Is the Pisound more sensible than the Pi itself to voltage variations? I know that the Pi has a warning circuit for undervoltage detection.