It seems that the pot (channel 6 = A1) introduces some noise in channel 7 (A0) conversion. You could try to suppress this noise this way:
1/ Replicate the exact same schematic as in the youtube video (this one works fine), and the exact same code as the one showed in the video.
2/ Add a pot to A2 just to produce a noise and see if the sound is still correct or not,
3/ If you can hear the noise, I guess this is a sort of EMI and you should convert this noise from both A0 and A1.
Do analogin = analogRead(A0) – analogRead(A1); This should suppress most of the noise. See if the result is correct.