In Rythmbox, if you go into Preferences/Music and then the Library structure heading you can set your preferred format to Mpeg Layer 3 audio (mp3). This or the Mpeg Layer 3 setting cause an information popup.
"Install additional software required to use this format"
just wondering your just trying to listen to your music? if so i use strawberry its an awsome music player and its in the software store. worth looking at....
When You open the Software Store, scroll down to the End, You will see there a Button called ''Codecs''. When You click on it, You get an Overview with installable Codec Packages.
Then You can install more Codec Support over the Terminal with:
vlc is a Media Player which brings Codec Support with it. ffmpeg is no Player but brings Codec Support, too. ubuntu-restricted-extras is a Packages Bundel which brings Codec Support and Microsoft Fonts. That is the Reason why You will get a Licence Agreement Prompt where You have to navigatewith the Tab Key to ''Ok'' and press Enter to accept.