@DonDodda Please check if you installed the "More Tracks and Formats"-add-on from the in-app store.

This is not included in the free version of Music Maker. If you need to buy it and don't want to, you can also use a converter and convert your MP3 to WAV which can be loaded by Music Maker Free Edition. XMediaRecode is a good free converting app. It can also convert many other file formats. https://www.xmedia-recode.de/en/