BlueCrab has successfully hacked the Dreamcast microphone, he posted in this topic at dcemulation.com
<blockquote>
Well, this evening (heh, evening), I've managed to successfully hack together some code to access/use the Dreamcast Microphone (the one you'd get with Seaman). More info will be forthcoming in the morning, when I get up.
I'd like to leave you with this little sound clip I recorded off my mic: http://www.ljsdcdev.net/dcmic2.ogg
This sound clip is what I believe to be the 2nd ever thing recorded off of a dreamcast mic using homebrew code.</blockquote>