I also have this problem, and I've followed these instructions and I can't get them to work. I have had Mic Boost working before, but when I had to roll back the registry and settings on my computer, it was lost. I remember having the hardest time trying to find it.
Anyway, here are the specifics.
I am using realtek audio HD input as my "mic" (it's a soundcard default, apparently). I am running Win XPSP2. When I open the advanced volume controls and go options, the advanced controls setting is greyed out. Also, all the sliders under recording except the master control are locked. In short, like the above poster.
I can't find the mixer.exe, so if that's the only way, could someone post a path?
EDIT: Ha ha. Here's one way how to do it if all your options are greyed out:
Go to the Advanced Volume Controls under output.
Then go down the list of available sliders that can be shown.
If you're using the front port, then go down to Front Pink (or whatever colour) In
If you're using the back port, go to Rear Pink In
Check the box
Then go to the normal output meter on Advanced Volume Controls.
Go Options--->Advanced Controls.
Then click the Advanced button under Front (or rear) Pink In.
At the bottom of that window, click the Mic Boost option.
It should now work