There is a way to do this on a Mac:
1. Install a utility called "SoundFlower" (
http://cycling74.com/products/soundfl...)
2. From the Apple menu, select System Preferences
3. From the System Preferences window click on the Sound icon
4. Click on the Output tab/button and select "Soundflower (2 ch)"
5. Start a GoToMeeting meeting
6. In the GTM interface, click on the microphone icon and, from the menu that appears, select "Soundflower (2 ch)"
Now, Any audio that would ordinarily be outputted from your Mac's sound system (like from the QuickTime player, iTunes, etc.) will be channeled directly through GoToMeeting.
It should be noted that this is a technique that you'll want to use sparingly during a presentation as switching back and back and forth between audio sources is a little involved: When you wish to speak, you'll have to click on the Microphone icon in the GTM interface again and select "Internal Microphone." If you wan't to hear what attendees are saying, you'll have to change your audio output through the Mac's system preferences back to "Internal Speakers."
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Dale