Hi Jack.
I ran your suggested test, but first, I must correct an error of mine. There is playback within Youcam. I didn't spot it because their is nothing to spot, no button, menu, right-clicky, nothing. I missed the obvious "Double click the created thumbnail" scenario, which launches your movie in a built-in viewer. Nobodies perfect.
My first thought was to record silence in Youcam, play it back and record again. No joy. The Youcam player opens in a modal window, so no concurrent tasks. The fallback position was record silence, playback in WMP while recording a new file in Youcam, so here we go with what I found.
It is impossible to record silence with a live mic. The best you get is effectively white noise and lots of it, coupled with some high freq elements which I found to be generated by the hum and noise of the PC itself. If flaccon thinks she can record silence, she is plain flat out wrong. At best she can record the white noise that is around everyone.
In any event I proceed to run the recorded silence through WMP to get to the next step, but paused immediately. The playback through WMP and Youcam were audibly different. Playback through Youcam was a mostly consistent white noise. Playback through WMP exhibited a low frequency harmonic...something. VLC showed the same low freq harmonic. Youcam didn't wanna. Suggests to me some codec issue perhaps.
Next step, record another while playing back the first. Basically, the harmonics simply got worse.
ETA: Good grief, reading back my post I discover I am writing the script for "The Hunt for Red October" ROFL.