Just don't try the Griffin Technology's iMic. I already tried and it doesn't work under OS9.x or earlier. There aren't many USB audio interfaces that work with Classic Mac OS versions, unless you count digital cards from M-Audio.
UPDATE: Well, it appears the headphone jack itself has cracked on the plastic housing, due to a combination of the cord snagging on Sunday and also old age. I have two choices at this point: (1) obviously replace the DC-In/Sound Board (about $80 USD, not including tax, S+H, etc.) or (2) rewire a new stereo 1/8" jack with a pigtail to the connections on the old DC-In/Sound board. The latter is much cheaper and I know it should work. Apparently iFixit.com is out-of-stock on the board. I'll look around elsewhere, too.
UPDATE II: It appears Apple uses a stereo 1/8" jack with a built-in N.C. (normally closed) switch, so that the Mac can disable the built-in speakers whenever you fully plug in a pair of headphones. So now I have to check and see which 2 of the 4 solder joints is the true ground for headphones. The replacement jack I have doesn't contain a built-in switch, so I'll have to add additional external wires and a simple switch to manually activate the jack. 2 of the 4 solder pads are closely tied in with the left channel. When you "close" the circuit between these two points, it tells the Mac to feed sound only to the built-in speakers; not even changing settings in the control panel will do anything. If you add a switch, it sends a signal triggering that sound should go through the headphone jack to the headphones.
73s de Phreakout.
UPDATE: Well, it appears the headphone jack itself has cracked on the plastic housing, due to a combination of the cord snagging on Sunday and also old age. I have two choices at this point: (1) obviously replace the DC-In/Sound Board (about $80 USD, not including tax, S+H, etc.) or (2) rewire a new stereo 1/8" jack with a pigtail to the connections on the old DC-In/Sound board. The latter is much cheaper and I know it should work. Apparently iFixit.com is out-of-stock on the board. I'll look around elsewhere, too.
UPDATE II: It appears Apple uses a stereo 1/8" jack with a built-in N.C. (normally closed) switch, so that the Mac can disable the built-in speakers whenever you fully plug in a pair of headphones. So now I have to check and see which 2 of the 4 solder joints is the true ground for headphones. The replacement jack I have doesn't contain a built-in switch, so I'll have to add additional external wires and a simple switch to manually activate the jack. 2 of the 4 solder pads are closely tied in with the left channel. When you "close" the circuit between these two points, it tells the Mac to feed sound only to the built-in speakers; not even changing settings in the control panel will do anything. If you add a switch, it sends a signal triggering that sound should go through the headphone jack to the headphones.
73s de Phreakout.