>> Well, I haven't tried it myself, but the EXT IN pin (pin 26)
>> is connected to the audio in pin (pin 5) of the A/V connector.
>> You can mix external audio to the SID output and, most
>> importantly, run it through the filters. It's also possible
>> to daisy-chain multiple SIDs like this.
>
> I've heard that it is really easy to fry your sid with this kind of
> experiments, so take care!

Good point :-) The forementioned appendix has
some details about the signal it should accept.
Notice the *should*, I haven't tried :-)

Stakker