The package that is providing ALAC support on Xenial (16.04.6 LTS) desktop is:
I was able to confirm this by removing:
and autoremoving its (orphaned) dependencies:
and then trying to play my ALAC .m4a files again. Rhythmbox did a bit of searching and popped up a dialog that said:
Install extra multimedia plugin?
Rhythmbox requires to install plugins to play media files of the following type: Apple Lossless Audio (ALAC) decoder
libav plugin for GStreamer
[ Cancel ] [ Install ]
As far as I can tell, Ubuntu Touch (OTA-10) stable does not come with this package installed. I guess my next step is to do a bunch of reading to figure out if I can install this package myself, and/or request that it be included in a future UT release.
It's possible that gstreamer1.0-libav brings with it a bunch of baggage that might negatively impact UT...or something. Beats me.
Working this out has been fun (so far). Cheers!