This happened to me as well...
1. right click on the one you want to record on.
2. scroll down till you get to "Set Recording options"
3. choose 'channel' and change to 'microphone'
4. notice your 'record vol' has become active....
There are other ways as well, if it has been recorded already, then on the bottom is the master volume, however, remember if it is too far in the red, you are going a bit "too hot' and may distort some of the sound.
I was trying to link some pics I made, however, this BB won't allow it...
Hope this helps,
JG