how to convert stereo to mono

I believe that you can use the free download, Handbrake, to convert the clip from stereo to mono. At least, there are settings that appear to allow that.

You might also go to Applications/Utilities/Audio MIDI setup.app, click on your output device, and then click on configure speakers. You will get a window like this:

I'm thinking that if you set both speakers to "1" that might give you mono. However, that might be just a speaker setting. I don't think that it will convert the clip itself, and I don't know whether it would convert the stereo clip to mono on export. I've never tried this myself so you would need to experiment.

-- Rich

Why do you need to convert stereo to mono?

Stereo is basically 2 mono tracks recorded from widely different positions (at least 2m separation) but so-called stereo mics on cameras etc. are so close together that they record essentially 2 identical mono soundtracks.