If you are coming up against an audio output not being captured possibly one of the following scenarios might answer your questions why.
Which scenario are you coming up against:
1. There is a mic and headset plugged in and you want to play audio from the computer.
If yes, than you need to unplug the headset so this input can pick up the audio from the speakers.
If no, see next scenario.
2. There is just a mic with no headset and you are playing the audio from the computer.
If yes, we suggest you turn up the speakers and bring the mic closer to the speakers to pick up the audio.
3. There is a mic and or headset and you want to blend the audio from this input and computer output.
If yes, this is an advanced feature most capture tools don't offer. We have this on the road-map.
If non of these scenarios are happening, you might want to look at the following article that might address your question. Otherwise, feel free to contact support, we would be happy to assist you.