I found my 5D defective after owning since last April. So Canon will fix it because I’m still under warranty. But I’m letting you know so you can check your 5D Mk III.
According to Canon the HDMI cable should carry audio from any video files being played back in the camera. Mine doesn’t. Nothing. Nada.
Canon tech rep courteously suggested I send it in for repair. I said I didn’t want to be bothered. That my Mac is also supposed to carry audio over the HDMI cable but doesn’t. So, I told him I thought it was in some sub-menu somewhere. He said it wasn’t. I courteously thanked him.
I didn’t want to lose my camera for a week and get a note when Canon returned it that “nothing was wrong.”
So, I called Canon again. Got a nice chap. He actually took the 5D Mk III he had at hand, found an HDMI cable, walked across the building to find a TV monitor with HDMI in, recorded a short video, played it back and told me “Yes, you should be able to hear audio coming over the HDMI cable out of the camera when playing back a video file.”
So I’m sending it in to Canon.
Be sure and check your HDMI out on the Mk III. If your’s doesn’t transmit audio, send it in while you’re still under warranty. If you need to send it in go to “Product Repair” on the www.usa.canon.com site. They’re cordial and prompt.