@hackman Thank you! Indeed, installing AMR plugins from gstreamer-plugins-ugly (as well as opencore libs they depend on) did the job and .amr files are now recognized and played back.
However, it is still impossible to play MMS attachments in Multimedia app directly from the message itself, because - apparently due to Sailjail restrictions - Multimedia app cannot access files in ~/.local/share/commhistory/data/ (i.e. where MMS attachments are stored). It takes manually copying the amr file to e.g. ~/Music for the Multimedia app to be able to open it (it doesn’t even work from ~/ but it must be one of the subfolders like Music).
Is there a way to whitelist a path so that Sailjail’d apps can access it?