Due to copyright concerns, CentOS does not support MP3 out of the box, so you need to install a third-party repo to handle it.

First, install a basic extension repo; you need at least one of repo, repoforge, or EPEL.

Then install the nux-dextop extension repo:

Bash
yum localinstall -y  http://li.nux.ro/download/nux/dextop/el7/x86_64/nux-dextop-release-0-5.el7.nux.noarch.rpm

If that URL is unavailable, you can browse that directory yourself.

Then install GStreamer:

Bash
yum install -y gstreamer-*

From my experience, you can get it done with wildcard matching directly; installing packages one by one also works, but there are only so many prefixes. The only downside is that the nux repo here is painfully slow.

At this point, the default Rhythmbox should be able to play MP3/MP4 files. If you want, you can also install my favorite player, Audacious.

Bash
yum install -y audacious