OK!
This was the problem I was having with multiple audio tracks in mp4 containers. Although a little kludgy, this solution would work fine for me (choosing the active stream from the ffdshow or Haali tray icons). However....
Those tray icons were not showing up when I played the files. I checked the settings for both ffdshow & Haali, and both have their tray icons turned on. I DO see the tray icons when I'm playing audio files. AAC-encoded files (m4a extensions) show both the Haali splitter icon and the ffdshow audio renderer icon.
A restart of MC12 didn't help, neither did a reboot of the machine...
When I investigated the MC12 DirectShow settings, I stumbled on a couple of issues.
First, 'mp4' was not selected in the 'DirectShow Filters' area of the options. Checking that box brought the ffdshow audio/video tray icons to life the next file I played, but I still had no choice of audio streams.
Second, I delved into the filter selections for mp4 files. I chose "Haali Media Splitter" as my source filter, and left everything else as the default. The next file I played showed all 3 tray icons, and the Haali icon allowed me to choose the audio stream. Success!
My remaining question is this--is there a tutorial about DirectShow and how all the myriad of MC12 settings affect it? I think I understand the basic concept, but I'm befuddled as to all the options. For example, in my case, with DirectShow 'off' for mp4 files, why was I able to play them back at all? What's the difference between a source filter and 'other filters'. Why would I want to choose multiple filters for a given file type, etc. etc.
Any knowledge will be gratefully accepted!
Brad