|Jonathan, Source Channel Mapping must be done before you use any portion of the clip in the project. Once the clip is placed on any timeline all options in Source Channel Mapping are greyed out.|
So using Source Channel Mapping is the very first thing you have to do before using the clip in your edits . Source Channel Mapping is indeed a very powerful feature and very useful one.
1. Import the clip and select the clip
2. Go to Clip > Audio Options > Source Channel Mapping
3. Select Mono
4. Click Ok
When you bring your clip down to the timeline there will be: Video+Mono Left+Mono Right. Two new mono audio tracks will be created when you bring the clip down to the timeline and you have full control over the audio via the Audio Mixer.