Thanks for your post. The main thing is that you've sorted the problem, but it's useful to know what caused it so others can benefit. Knowledge is power!

Trying to figure it, as you say, - most computer microphone inputs are designed for stereo mics and so have a miniature stereo jack socket. If you were using a single mono mic through an adapter, it's possible that you've grounded one side and made the other side common. Exactly why the input effects the output, I'm still working on! Perhaps it's to do with the ground being common to both. Perhaps some things aren't meant to be explained!

ROG.