Hey thanks everybody who replied. Turns out vvv was right. It is indeed a retaining clip for a jack. One of the 1/4" jacks, to be precise. Dug pretty deep into the unit to figure it out. Basically ended up rebuilding the fucker. It was due for a good cleaning anyway. Thanks shedshrine for the links. I'm usually not one to take electronics apart because I really don't know what I'm doing, but I couldn't stand not having this thing in working order. If it would have been just the one clip rattling around in there, I might have let it go, but I heard other stuff bangin' around, so I figured I better try to fix it before I plug it in, short something out and blow it up.
Split it open:

First clip was laying on the silver shield thing covering the guts of the mixer (on the right.) Other stuff was rattling under there. Better take that shield thing off:

Another clip laying there on the back side of the board in contact with the solder points. Still more rattling coming from under there. Separating the mixer guts from the plastic cover:

Bazinga! Retaining clips they are:

Metal plate covering the PC board needs to come off to get the rest of the loose shit out of there:

Five clips missing. For the life of me I could only find four of them. Must have made it's way out of the unit somehow...?
Clean it up and put it back together, hopefully ready to rock. Will have to wait to test it, getting late.
