wailing feedback

[Ron Reames]

When I turn up my volume I get a loud, high-pitched feedback wail from 
my 12" 600mz iBook.
Nothing I've tried- other than turning the sound down- will eliminate 
it.

Seems it started after I plugged in a cord into my external speakers 
port which was connected to a cassette player's speakers output.

I know I shouldn't have done an sound output to a sound output, but I 
had the laptop at a friends house and he used this method along with 
Spin Doctor to record a record to my mac.

Is some sort of sound hardware ruined on my iBook now?

Ron