well i had a similar probelem.....
ithe screaching sound wouldnt go even after a successful connection@!!!
my modem worked fine untill i upgraded to winxp.....
i didnt have the drivers for my connaxant internal 56.6k modem ......so i downloaded generic drivers......
and thats when the probelem started.........
i tried all the possible software solutions....none of them helped!
the only solution i had was to take the modem to an engineer and i asked him to disconnect the speakers......