So sorry if this is the wrong forum, but I’ve looked everywhere. I’m trying to play my laptop audio source (the audio from Windows Media Player, for example) through my same “audio out” that my USB Microphone is sending. Not in a multi-track recording environment, but live. To be more specific, I speak on Webinars which are live Internet connections where I’m talking on my USB mic/headset. Attendees can hear what I speak, but not my PC sounds (I’m sure by design). But I WANT to play things like music or the audio from a video. I’m not actually recording or editing anything. Any suggestions or direction would be greatly appreciated! 
I will admit there is the temptation to ask you why this is an Audacity problem.
But I won’t.
I would separate presentation from Web Services. Everybody always want to cram as many different services into one little laptop as they can. It’s never pretty and we get the people who exceed the capacity of the system.
The presentation should be done on a nice laptop and the audio output jacked into the machine hosting the web service. If you don’t do that, you will invariably get too many audio pathways running at once and start getting feedback or echoes in the show. The other option is the web service itself will actively preclude you from establishing the pathways you want.
See: Trying to record both sides of a Skype or Vonage call.
Let’s see if I understood the problem.
Koz
The problem is something similar to an issue that comes up with Audacity from time to time - “How can I record my USB microphone and MP3s/CDs that are playing in my media player for a live podcast recording?” The answer to that question is that you can’t. The reason is that Audacity can only record from one audio device at a time, so that means either the computer sound card or the USB microphone, but not both at the same time.
The same is probably true of your Webinar software. Unless the program supports multiple simultaneous devices, then you will not be able to have both the USB microphone and your computer sound system at the same time.
If you use a conventional microphone (not USB) that plugs into your sound card, then you may be able to use “Stereo Mix” as the recording device and thereby have both your microphone and your media player at the same time.
The other solution may be to play your media player through a loudspeaker placed close to the microphone so that the microphone picks up the sound from the media player.