This tool reads your M4A file directly in the browser and encodes a MP3 copy, so there is no upload, no queue and no waiting for a server. It works on desktop and mobile.
M4A audio from iTunes and iPhone voice memos is not accepted by every player, car stereo or upload form. MP3 is supported nearly everywhere.
Converting between two lossy formats loses a little quality, so use 192 kbps or higher.