I do a lot of LP and cassette conversions myself, and if I was troubleshooting this on my rig I'd start by digging my old cassette deck out (preferably one with VU meters)and swap the stereo lines from the computer's Line-in jacks to the cassette deck's. If the VU meters showed a reasonable signal and the cassette deck properly recorded a signal, that would show you have a good source.
Next, I'd plug the headphone output of a known good cassette player (or MP3 player)into the computer's Line-in port, try to record with CEP, starting with low volume and gradually increasing the player's volume.
An ordinary voltmeter/multimeter probably won't help. If you had access to an oscilloscope, that would do it -- but if it was me I'd probably try to use a cassette deck with VU meters!
My computer has on-board audio support, too.* It doesn't happen all that often, but it is possible for components to go bad, connections to break, even jacks and plugs can wear out.
* In all honesty, the on-board audio was too noisy for me, and I don't use my computer for critical recording anymore.
Edited by Capn Easy, 17 February 2011 - 07:29 PM.