How to record sound from videogame onto a sound file?

Hi, short question there.

Does anyone know a method, or program if I wanted to record the streaming sound from a videogame (e.g. ingame music in MxO), and save it as a sound file on my PC?

IrfanView has an option to make a photograph of what is currently shown at the monitor - is there something according with sound, where I can record any sound coming from my PC for a certain time?

Thanks in advance.