Line input perfect - mic (using same socket) V. low

I'm running Vista Home Basic & was using the mic input for skype & occasionally recording & it was fine. I don't remember what happened but I noticed the people I were talking to on Skype found it very difficult to hear me. I'd almost be shouting.

I've checked all the relevant settings under "recording devices" eg. "levels", "mike boost" & have checked that I've the most recent driver for Sigmatel High Definition Audio Codec.

I've also reset the defaults under the "advanced" tab.

I've used "BIOS" & have checked that the mike is working.

The input is physically ok as I've tested it by running a lead from the headphone socket to the mike input & the recording level was perfect.

(Sound output is fine in all ways.)

Please suggest what I can do to be heard again. Tx.