How can I convert Catalina music files on SD card to an iTunes library running Mojave?

I HAVE A 2012 MINI AND IT RECEIVED CATALINA UPGRADE AND DID NOT WORK WELL AT ALL, MANY ISSUES, SLOW WOULD NOT OPEN PAGES. WITH THE HELP OF THE GENIUS BAR I SAVE MY DATA (PHOTOS, DESKTOP,AND MUSIC) TO EITHER A CD OR SD CARD. NOW WITH MOJAVE THE MACHINE ACTS FINE EXCEPT I STILL CAN'T OPEN PAGES. THE HELP AT THE GENIUS BAR MISTAKENLY THOUGHT OS 14 WAS OS10.14. SIMPLE MISTAKE OK. BUT MY WORST PROBLEM IS I HAVE 2GB OF MUSIC THAT THE MOJAVE OPSYS DOESN'T RECOGNIZE ON A SD CARD. IT SENDS A MESSAGE TO me chose app at store??

HOW CAN I CONVERT THEM?

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