M4A is AAC audio in an MP4 container - what iTunes sold, what Apple Music and most podcasts ship, and what a voice memo becomes. At the same bitrate it sounds better than MP3, and it is read by rather less hardware.
Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a M4A file above and choose afterwards.