Here is good article that I found on enabling stereo mix. This is for Windows 7, not sure what system your on.

But turning this setting to enabled will allow fme to pick up sound from the game, you will just need to edit the xml file with what sound device to use.

Here is an example of my xml file for fme (the part with sound device to use)
<audio>
<device>Stereo Mix (Realtek High Defini</device>
<crossbar_input>0</crossbar_input>
<sample_rate>44100</sample_rate>
<channels>2</channels>
<input_volume>100</input_volume>
</audio>

Here is the link to enable stereo mix:
http://www.howtogeek.com/howto/39532...-record-audio/

If you need any more help / different system or what not, just let me know.