A few things to mention about the audio input pin:
If you want to work with this ext input pin, don't connect a signal directly! Instead, couple it with a 1-10 uF capacitor.
This pin is left floating in most (perhaps all?) C64 board revisions. It contributes to a huge amount of background noise and simply grounding it will give you a much better signal-to-noise ratio on your SID.
A switching jack can be used to automatically ground the pin when you unplug your input cable.