Let's start with #1:
How to filter on the number of audio channels?
MC has several features which you can use to do this. One way is to go to the search box at the top and type in something like:
[channels]=6
This will show you all of your 5.1 channel files. Or maybe you want to see anything that is *not* 2 channel. You could use this search expression instead:
-[channels]=2
The "-" out front says "search for everything except for this". So that would show you files with 4 channels, 5, 6, etc, but not with 2 channels.
You can also use a Panes view for this. Panes views have selection columns up top and then a list of files at the bottom. By clicking values in the columns, you can narrow down which files you want. If you add a column for "channels", then you can click on any of the values in that column to select only files with those values.
Panes can also hold saved searches. So one of the searches I typed above can be put into a column of its own. You can make as many of these as you want. Then when you click on that value, it will only show you files that match the saved search. This is very powerful and helpful.
I hope this gets you started.
Brian.