You probably need to install gstreamer1-plugins-mpg123
When installing py311-quodlibet from the command line, at the end it says:
Message from py311-quodlibet-4.6.0:
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Consider to install the following ports to support additional audio formats:
* multimedia/gstreamer1-plugins-faad : AAC audio support
* multimedia/gstreamer1-plugins-flac : FLAC audio support
* multimedia/gstreamer1-plugins-mpg123 : MP3 audio support
* multimedia/gstreamer1-plugins-ogg : Ogg audio support
* multimedia/gstreamer1-plugins-speex : Speex audio support
* multimedia/gstreamer1-plugins-vorbis : Vorbis audio support
* multimedia/gstreamer1-plugins-wavpack : WAV audio support
* multimedia/gstreamer1-libav : Many more audio (e.g., WMA) support
Some Quodlibet plugins require additional ports to be installed:
* audio/py-musicbrainzngs : for Musicbrainz plugin
* devel/py-pyinotify : for audio library update plugin
* www/webkit2-gtk3 : for lyrics window plugin