Hi Neil,

> what are they
>
> There are two unconnected pins on the PCB. What's up
> with that

That sounds like jumper 'JP2' at the top edge of the board: when the
little jumper is present, it shorts out capacitor C1, which allows the
module to process low-frequency, control-type voltages; when not there
the module will only work with higher-frequency, audio-type of signals.
I would normally expect it to be there unless the user has a specific
reason for wanting to take the DC level out of the signal input.

Hope that helps.

Tim