Huh, I just wrote a lengthy reply and it disappeared after trying to post!
I am cursed!
Anyway, there is no corrosion. Inside looks pristine. The cable is the same stereo one that has worked for 7 months. No degradation of sound at all, just suddenly stopped working.
There is no noise at all when ...

Background:
I got a late 90's purple Fly around 7 months ago, and of course I love it.
I traded in a Nitefly to get it.
When I went to pick it up, the piezo didn't work. Stereo cable, good battery, etc. all things were checked.
I came back twice and eventually they got it all to work right. They ...

Excellent!

This is exactly what I needed. Very useful.

( ahhhhhh so THAT"S what that knob does....)