I actually know what you're talking about, because I had this occur after I switched to a new sound card. It seems to only happen in certain circumstances with a particular system setup (like mine, but it was only after I changed the sound card). This doesn't seem to actually an issue with my app in this case, but an issue with windows that I don't quite know the source of. When mic capturing starts, it just for some strange reason decided to reset all the volume levels to the levels that my previous sound card had used. When the mic is set to disabled the problem doesn't seem to occur though, it's very weird.
I did finally manage to fix it, but I had to cleanly uninstall all my sound drivers and reinstall them. I still don't know the exact cause, but I'll see if I can't find some more info about this on google somewhere.
If I can't find more info, I may be able to fix the problem in some other way, maybe. I may be able to force the levels back to the values they're supposed to be at.