What is the best way to combine two audio sources into a single audio output?
I misspoke in my original question, I've fixed the sentence so
  hopefully it should be clearer. I'm just wanting to listen to the PC
  out, while simultaneously listing to the Mic out as I record it.


If you using Windows, then just go to the

 Sound (Right click in notification area) > Recording devices > Chose you device > Properties > Listen > Listen to your device

That would allow you to hear your recording device input in your playback device(speaker).