Hi Tom, we made some test experience for playing youtube (online) or mp4 movies (prerecorded) during a webmeeting.
The problem was that the video is played to all attendees, but the audio can be heard only on the presenter computer.
Not all computers do have an audio hardware that allow to intercept an audio stream and to redirect it as source for another program running on the same computer (in that case, you could get the audio redirected to webmeeting input simply selecting the appropriate source).
So we tried third party software mixers (easy to find googling around), with varying results.
1. they consume a lot of cpu; if your pc is not strong enough, you'll get choppy video and stuttering audio
2. their configuration it is not so easy to manage for an average user
3. difficult to mix up different sound sources, as mic and playback, with appropriate levels
4. attendee's audio returns back to them, if any noise came in.
Too many drawbacks.
Our decision was not to disclose this to our customers.
Regards
Carlo