I am looking to combine the system sound with an external mic plugged into the microphone jack and send the combined audio output to HDMI. Is this doable on my Macbook pro. I have tried different things with sound flower and creating aggregate devices and such without luck. I am able to create a recording in Quicktime which has the sounds sources mixed perfectly. But, I want to be able to send this output live through HDMI. Any help is greatly appreciated.
If you are looking to combine separate HDMI video and audio into a single HDMI connection (to go to a monitor, for example) you need an HDMI Audio Injector
This device is separate from your MacBook Pro and will work with virtually any digital video source and either an analog or digital audio source.
As for getting the sound to/from your MBP, you will need a Mini DisplayPort to HDMI for the video and perhaps a USB Audio Device. Here are two products I have personally used with good success:
You hook the outputs up to the HDMI Audio Injector and you should get the results you are looking for.
I hope that helps.