Here we go-



No plug inserted

Headphones/Audio Out

- No pins are connected. Pin 1 conducts to the thread (ground?) on the jack.

Mic/Audio In

- 3 connects to 4

- 5 connects to 6

With plug inserted

Headphones/Audio Out

- 1 connects to 2

- 1 connects to 4

- 2 connects to 4

Mic/Audio In

- 1 connects to 2

- 1 connects to 4

- 2 connects to 4

- 5 connects to 6

Disclaimer: My heavily corroded parts could be faulty.

Conclusions: It does appear that an open speaker jack switch = no plug inserted. The connected pins on the line in shouldn't make a difference to audio output, should they? Besides, they conduct, so presumably they were working when they were on the logic board.

These results, coupled with the FPU issue, the visible corrosion, and brief message board surfing here:

http://www.megadon.co.uk/wb/whiteboardshow.pl/syrinx/wbfile=Mac%20Classic%20Repair.wb/page=default.html

http://www.mail-archive.com/vintage-macs@googlegroups.com/msg02082.html

Leads me to think that washing is the next step, with recapping after that if no improvement.