There should be a man page (
man omxplayer); the options are also explained here. I do not see a "background" or "damonize" (similar concept) option there.
This is probably because
omxplayer was written specifically for the pi to exploit hardware acceleration for video playback (it may be less unique now in this sense than it once was); it will play audio mp3's simply because (educated guess) it needs to include that decoding/playback code anyway.
However, using the audio system on the pi does not require any special software written for it, so if all you want to do is listen to mp3's you can use anything that runs on linux. For a command-line interface that I think will background, try
apt install mpg123); I believe this should be set up to use ALSA as the soundsystem so if you are using
pulseaudio and don't hear anything, that's why (it could also be vice versa). Checking here (
man mpg123), you can list the possibilities with:
And use one, e.g.:
mpg123 -o alsa
It does mention being able to use a "comma separated list" there too (which probably means commas with no spaces).
Finally, if this is a big issue for you with
omxplayer and you are using a GUI, it is obviously no big deal to open another terminal or an extra tab in the current one and just let it run there. They cost very little in memory as most of it is shared. The same logic applies if you aren't using a GUI; on the real console (i.e., screen and keyboard), try cycling through
ctrl-alt + F1-F6. If you are using
ssh or something, multiple connections are also cheap (and there is the
screen option mentioned by PandaLion; see also