First, I add the second audio track in Subler and save.
Returning to iTunes, I'm not given the option to switch in iTunes or QuickTime 10.
So I go back to Subler and turn one track off, saving again.
Now I'm given the option to switch.
But the audio doesn't work. The standard track works fine. Switching to the other one makes a popping sound and then pure silence. Switching back works fine. This also happens in QuickTime 10.
So I tried to enable/disable via QuickTime 7 Pro.
Turning a track off forces me to resave the file, but not like Subler, oh, no. QuickTime 7 Pro forces me to reencode the whole thing as an MOV, which takes forever and is pointless.
I've done this without having to save before. What's going on? How do I add a second, selectable audio track that works on things in iTunes without QuickTime 7 Pro forcing me to save MP4s as MOVs and [insert any audio filetype with zero video whatsoever here] as MOVs as well?!