I don't know if Music Maker supports MP4a files. MP3 files are fine but it's not supported by the free version. You either buy the More Tracks and Formats option from the store or if you have the Plus or Premium version you add this as one of your free options. To use them you put them in a folder and find them using the file manager tab.
Many thanks Graham. I will check out these options asap. As an amateur musician I have been using, albeit infrequently, the 2013 version and downloading files to work on was very simple!
. . . .if you have the Plus or Premium version you add this as one of your free options . . . .
The More Tracks and Format as well as a lot of other features, Soundpools and Instruments are already added to your account when you activate the Plus or Premium version.
These should be downloaded and installed before using up any free extras, then choose carefully what free extras you need as you cannot change your mind once you have selected and downloaded them.