Not everyone knows that you can configure your recording properties in such a way, that you broadcast sound over a FlashCom connection even when you haven't got a microphone connected (ok, you can't broadcast your voice but pretty much everything else). I am not sure as to how much this setup applies to Mac's but here is how it's done on WinXP:

First of all right click on the Flash movie running the FlashCom application over which you want to broadcast. From the popup menu that appears select 'Settings..'
The Macromedia Flash Player Settings Panel will appear.

At the bottom of the panel select the Microphone symbol.

You should now see a dropdown menu which contains all available sound input devices on your machine. If you have a webcam with a built-in mic connected it will show up here. A standalone mic also.

However we want to select the computers soundcard. On my machine it is called SOUNDMax Digital Audio - yours will probably be a different one. Select it.

Your soundcard is now selected as your input for FlashCom. All that is left to do now is to tell Windows that it should use this settings in its recording properties which in turn will enable us to broadcast this source.

Double click on the little speaker icon which sits at the bottom right on your task bar. The volume control panel pops up. (alternatively go to start>settings>control panel>sounds and find the audio control panel, you can then go straight into your Sound Recording properties)

Select Options > Properties.
On the properties panel you will have to click the radio button which reads 'Recording'. Next select your sound card as the Mixer Device from the dropdown menu. This should be the same name as the one you have previously selected on the Flash Player Settings panel.

And that's it. You might have to change your recording volume settings as you cannot broadcast any sound if the output volume is set to zero ;-)

It is usually either 'Mono Out' or 'Wave Out Mix' that needs to be turned up.