No, I probably don't.1) It depends upon whether you want a separate display for each channel or not.
When the music is not playing, it sounds a bit like modem noise. Digital noise I would say.2) Jumpering as you indicated should work. What type of noise?
Someone from Sparkfun told me:
.As far as the modem-like noises, that seems like you are probably getting feedback introducted into the audio line with the addition of the second audio jack
Also what does this mean on the datasheet:
The multiplexer readout rate also controls the decay time (10% decay per read), so no external pins are needed for this function.