f you do a search on this forum you should find several threads about this.
I must be search-challenged, because when I searched for "recorder mixer" I got (besides this thread) one somewhat relevant thread, but it didn't tell me anything more than MC's help file does under "Record Web Media". And the help file's advice doesn't match well my setup.
I'm running Windows XP and SB Live! Wave Device. From Windows audio control, I went to Options | Properties and clicked Recording. Help file at this point says
The Master Volume box appears. If you have a source called "what-u-hear", or "sum" or "all", select (or un-mute) that source. If you don?t have one of these, try the others until you find the one that works (Stereo Mix may work). I don't have those. My control panel shows several inputs (MIDI, CD Audio, Line-In...), and I can select one of them. I select the "Wave" input. In MC's Recorder, I select "Mixer" as the recording source. But when I play something in RealOne, nothing gets recorded. Just out of curiosity, I tried selecting each of the other devices, but none did anything for me.