I have a new MP3/AAC player with some problems reading the metadata on
some of the m4a files, so they come up "Unknown Artist" and "Unknown
Album", even though I've made sure the tags are correct using easytag.
The file command gives
ISO Media, MP4 v2 [ISO 14496-14]
for the ones the player handles correctly, and
ISO Media, Apple iTunes ALAC/AAC-LC (.M4A) Audio
for the ones the player mangles.
I may have ripped some of the files with grip a few years ago, but the
recent ones are all ripped from CDs using abcde. I have the faac and
libfaac0 packages pinned to version 1.28-0ubuntu2 because later
versions were effectively broken for licensing reasons.