this is a Windows issue, not FSX... you should be using Google to learn how to check/enable computer features
go to [control panel] >[Sound]> [Recoding tab]. If the mic is working you should see indications on the volume bar. But select the mic device and look at it's properties. It may not even be turned on...
Google is your friend.
It would even help a bit if you would tell what OS you are using; details matter.
Hooked since FS4... now flying: FSX Acceleration on Win7/64, Core Duo E8400; GA-EP45-DS3R; GTX 460-768MB; 4G RAM; Freezer 7 Pro