Help.. link from input to output lost..

Not sure who to explain this, but I have a laptop Dell with WinXP and an
on board Sigmatelaudio sound card. This computer has an onboard
microphone and a line-in input. Initially, if I had audio coming in the
line-in input, the audio would come out the PC's speakers or earphone if
connected, assuming that I had the proper volume controls enabled in the
mixer. However, I made a change to a registry item, to change the
default MP3 CODEC (I have since changed this to it's previous value),
and at approximately this time (ie I'm not sure this is what caused the
problem), the computer no longer sends the audio from the line input to
the output, ie I can no longer hear what is attached to the line-in on
the PC's speakers. I have recording devices, which can record the audio
on the line-in, and I can immediately tell this software to "play" the
recorded audio, and it plays properly through the speakers, but I can't
hear it live.
It is as if a "mute" parameter has been selected somewhere, but I
can't find any such parameter. I have all items in the mixer enabled and
at full volume, for both play and record.
Can someone suggest a possible cause for this, and aim me in the
right direction for correcting the problem?
Thanks.

--
Bill Jones Sweden Maine N3JLQ
htpp://www.megalink.net/~wejones TVRO,Space,Ham radio,WX
htpp://www.megalink.net/~wejones/wwii.html B-17/wwii/303rdBG