Check whether the M4A needs conversion
If the receiving app accepts your M4A, retaining it may avoid another encoding. When MP3 is required, inspect the actual codec first: AAC, ALAC and Opus have different quality implications despite sharing an extension.
AAC, ALAC and Opus take different paths
Qualified AAC-LC and mapping-family-0 mono/stereo Opus decode to float32; supported 16/24-bit ALAC decodes to matching integer PCM. The decoded output is verified before the MP3 encoder runs. AAC-to-MP3 and Opus-to-MP3 add another lossy generation. ALAC-to-MP3 starts from losslessly stored samples, but the MP3 output is still lossy.
Timing and extra tracks
The reader checks the declared presentation, including supported priming and end trimming. A tested music-export layout with one tx3g timed-subtitle track and recognized C2PA provenance is supported; conversion omits those extras with a warning. Video, additional audio tracks, encrypted files and unsupported layouts are rejected. Renaming a protected file does not remove DRM.
A recording or music delivery copy
Select the original M4A, confirm its detected codec and review the resolved MP3 bitrate. Check the beginning, ending and duration after download, whether it is a voice memo or a music track. See voice-memo export guidance and M4A codecs explained.