If you already can record ingame audio on your computer you don't need this. If you can't and other solutions haven't worked for you, try this;
http://forums.trainsim.com/vbts/showthread.php?296534-Dell-Sigmatel-and-FRAPS-audio-recording
http://forums.trainsim.com/vbts/showthread.php?296534-Dell-Sigmatel-and-FRAPS-audio-recording