I found a workaround. The problem is related to this picard/mpeg4ip bug[1]. The fix is to run the atomicparsley[2] program described in the bug on each file:
AtomicParsley file.m4a --freefree The latest version of Picard shouldn't have this problem anymore. [1] http://bugs.musicbrainz.org/ticket/1022 [2] http://atomicparsley.sourceforge.net/ -- gstreamer doesn't like certain m4a files (musicbrainz tags?) https://launchpad.net/bugs/48727 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
