Only a thumbnail of your schematic can been seen on edaboard unless the viewer is registered, better to post the image on here if you want people to see it.
To use a digipot with audio signals the signal must be biased somewhere close to the centre of the digipots operating range. If you are trying to pass an AC coupled signal though the MCP4131 then it will be clipped on the negative going parts of the waveform (effectively half wave rectified) which will cause a huge amount of distortion.