Does anyone know how I can properly connect an analog speaker-out jack to my digital camcorder's line-in?

TIA

Assuming that you mean the speaker out jack from a soundcard, then you need a stereo 3.5mm to RCA cable (e.g. from radioshack). Turn the volume control of the soundcard as LOW as it goes, connect the cable, then try a series of recordings with the soundcard's volume turned a bit higher each time until you find the point where the loudest sounds are becoming distorted in your recordings.

Or, since the camera is looking for a "line level" signal, find a soundcard that has a "line out" as well as "speaker out" connector, use the same cable, and simply bypass the soundcard's built-in amp (which is probably so-so quality).

Thanks, but actually the speaker jack is on an old Super-8 film projector, and appears to be a mono RCA connection. Sorry, I should have been more explicit.

In that case you need a standard RCA-RCA cable (maybe mono - stereo)...then just go through the same steps of starting with the volume real low and turning it up notch by notch...