M4A vs MP3
M4A usually contains AAC audio, which sounds better than MP3 at the same bitrate. MP3's advantage is compatibility: car stereos, older MP3 players, some editing software and many upload forms only take MP3. Converting is a lossy-to-lossy step, so do not go lower than the original bitrate.
Note that iTunes / Apple Music purchases and downloads protected by DRM cannot be converted.
Private conversion
Your file is opened by your own browser and converted on your device. Nothing is uploaded, so there is no queue, no file-size quota and no copy left on someone else's server. You can even disconnect from the internet after the page has loaded and the conversion still works.
Voice recordings
Recorded something with the Voice Memos app? See the step-by-step Voice Memo to MP3 page.
By Furkan Kaya, developer of ConvertEase · Updated