Audio in my video doesn't work on all computers

[MattMedia]

I'm making short videos for my work, using a Canon Vixia HF10 camera, an Audio Technica AT831 microphone, and am editing in Vegas Studio 9. A sample of the video is here: http://www.youtube.com/watch?v=QNFUD6zurZ8

Here's my problem - on some computers, the audio from the microphone doesn't work, but the music does. On the computers in my office (PCs), it works, on MacBooks it tends not to, unless they have external speakers, on my macbook pro and my wife's iMac, it's fine, and on my iPhone, it doesn't work with the built in speakers, but does work if I have my headphones in.

What's the problem here?

[jagabo]

Probably a missing center channel speaker and misconfigured speaker configuration. With 2 speaker systems the computer should be configured to downmix multichannel audio to stereo.

[MattMedia]

I'm sorry. I'm pretty new to this. How do I fix that?

[neomaine]

I'd save your audio in mono. I doubt anything else would be noticed anyway.

[jagabo]

Encode as mono or stereo.