It has been driving me nuts for several days trying to figure out how to control when the microphone is on or off on my Lenovo T60 laptop with Windows 7 RC (I am assuming the RTM is the same.) I have been listening to a lot of online training materials the last few days and the mic has been on the whole time with no obvious way to turn it off. I finally took the time to figure it out.
You have to mouse click on the speaker icon in the task bar, then click on the speaker icon at the top of the volume adjustment graphic that pops up. That will open a new window titled Speaker Properties, you pick the Levels tab and under that is Microphone. You can adjust the level of the microphone but more importantly there is the option to mute it (you click on the speaker - yes I know, it doesn't make any sense.)
Why the microphone mute option is buried here I have no idea, you can get to the same window by right clicking the speaker icon in the taskbar and selecting Open Volume Mixer then doing the same click on the speaker but this is just as un-intuitive as what I outlined before.
Given how many folks are doing multimedia stuff today (video/screencasts/podcasts) you would think the microphone would finally get some respect and its own icon set? Guess not.
Updated post for the RTM on this is at: