Can anyone help me with this vocal canceller circuit diagram?
2. What about the left and right input's ground must connect to the ground in the circuit ?

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Yes ground in this circuit is a common ground. SO the audio channel grounds need to be tied to the common ground. Same goes for the ground of the output.
Anyway to answer your next question you hook them up just like you see it. Your 9V power source connects to R7, R7 connects to R8, and R8 connects to ground. As you already know one of the leads on the pot connects to ground as well.
I'm guessing your still learning to read a schematic?
Thanks for teaching me !