I hear no interference so not sure what interference you think you hear. I do hear harmonics of the power supply in either your receive end or the transmitters end. You might remove that by filtering out something like frequencies under 400 Hz with an audio filter but you might not like how that sounds. At the same time, removing above say 3000 Hz with an audio filter might also help in some cases; but here, I think you will only make the audio less what you may find natural.