Created attachment 354504 [details]
rhythmbox.log

I'm on debian testing/buster, system is up to date.
Recently a couple of gstreamer packages got upgraded, namely: 
gstreamer1.0-plugins-ugly: 1.10.4-1 to version 1.12.1-1
and
gstreamer1.0-libav: 1.10.4-1 to version 1.12.1-1

Output of dpkg -l | grep gstreamer:
gir1.2-gstreamer-1.0                  1.10.4-1
gstreamer1.0-clutter-3.0              3.0.24-1
gstreamer1.0-libav:amd64              1.12.1-1
gstreamer1.0-plugins-bad:amd64        1.10.4-1
gstreamer1.0-plugins-base:amd64       1.10.4-1
gstreamer1.0-plugins-good:amd64       1.10.4-1
gstreamer1.0-plugins-ugly:amd64       1.12.1-1
gstreamer1.0-pulseaudio:amd64         1.10.4-1
gstreamer1.0-x:amd64                  1.10.4-1
libgstreamer-plugins-bad1.0-0:amd64   1.10.4-1
libgstreamer-plugins-base1.0-0:amd64  1.10.4-1
libgstreamer1.0-0:amd64               1.10.4-1

Since then rhythmbox and totem won't play mp3 files. Rhythmbox version is 3.4.1-2+b1.

System log shows this message:
Missing plugin: gstreamer|1.0|rhythmbox|MPEG-1 Layer 3 (MP3) decoder|decoder-audio/mpeg, mpegversion=(int)1, mpegaudioversion=(int)1, layer=(int)3

Same for totem:
Missing plugin: gstreamer|1.0|totem|MPEG-1 Layer 3 (MP3) decoder|decoder-audio/mpeg, mpegversion=(int)1, mpegaudioversion=(int)1, layer=(int)3 (MPEG-1 Layer 3 (MP3) decoder)

Running rhythmbox -d &> ~/rhythmbox.log shows those relevant messages (see attachment).

Mpv for example is able to play mp3s though.

It seems that removing the mad mp3 decoder plugin from gst-plugins-ugly, as stated in the release notes for gstreamer 1.12, is causing some applications trouble on my system. The note further says: "Use the mpg123audiodec element from the mpg123 plugin in gst-plugins-ugly instead, or avdec_mp3 from the gst-libav module which wraps the ffmpeg library". How do I tell rhythmbox to do that?

So far I omitted the problem by downgrading gstreamer1.0-plugins-ugly back to 1.10.4-1.