The idéa was to take a jack input, and then with some pot-meters/variabel-resistors on the multiplexer to the arduino, to fx. pitch or speed-up the sound.. Or whatever that can be done
Well you have 6 analogue inputs. If you use one for the digitising of the sound then that leaves you 5 pots you can connect without the need to multiplex anything.
As I said the memory is very short but a simple 256 byte buffer will allow you to do pitch up and down but the quality is not the greatest because of the fact you only have 10 bits resolution on the A/D.