In Rhythmbox's Preferences, you can change the "Preferred Format" for Music to MPEG Layer 3 Audio, Ogg Vorbis, FLAC, or MPEG 4 Audio. However, despite there being a Settings button, it does not become enabled for any of these choices. (I have installed all of the gstreamer plugins, but this has made no difference.) So how can you change the "Preferred Format", for example to change the bit rate or the quality setting?