I recorded episodes 9 and 10 last night. One of them used my built-in mic, which makes me come across loud and distorted; in the second, I tried my high quality mic, but I still couldn't get my voice loud enough to counterbalance the game. Someone helpfully suggested recording the commentary track separately from the game video, but that would be time-consuming (and would also deprive you of my spontaneous reactions while playing). Tired of these issues, I finally relented and purchased a set of headphones with an attached microphone. Hopefully, that will improve sound quality going forward.
One thing that's really great about doing this Let's Play is that I'm learning how to put together polished commentated videos. This will help a whole heck of a lot with my future efforts at marketing Sinister Design games online.