If you have a device that needs music in MP3 format but you’ve downloaded everything from the iTunes music store then you have a problem, in that the music is in AAC (.M4A) format and won’t be recognised.
The good news is that converting it is easy.
The following is how to do the conversion on a Mac using iTunes. Using Windows is similar however the preferences menu under File rather than iTunes.
The first step is to set your import settings to MP3.
This is done by selecting preferences from the iTunes menu
Then click on import settings and set as shown on the screenshot below
click OK, then click OK again to save the settings you have made.
Then create a playlist (cmd N) and drag the music you want to convert into it.
Select all the songs in the playlist (cmd A), then right click (cmd click) and select Create MP3 version
All the songs in the playlist will be converted into MP3 which can then be copied to your device.