AAC or ALAC?
M4A is an audio container in the MPEG-4 family, not a codec name. AAC is the common payload and is lossy: some information was discarded when the recording was encoded. An M4A can also contain ALAC, the Apple Lossless Audio Codec. That difference changes the quality history of the conversion. AAC to MP3 is a second lossy encode. ALAC to MP3 starts from lossless audio, but the new MP3 is still lossy.
Decide whether you need MP3 at all
If the original opens in the software or device you plan to use, keep it. Converting AAC to MP3 cannot improve it, and raising the MP3 bitrate cannot restore what AAC removed. MP3 is useful when the destination explicitly requires .mp3 or rejects M4A. PrivateMP3 writes MP3 at 192 kbps; there is no bitrate picker on this page.
What each output does with an M4A
| Source and choice | Actual processing |
|---|---|
| AAC → MP3 | Decode AAC and encode a new 192 kbps MP3. |
| ALAC → MP3 | Decode lossless ALAC and encode a lossy 192 kbps MP3. |
| Unedited AAC → M4A | Copy the AAC packets without re-encoding. The page shows a copy note. |
| ALAC or edited audio → M4A | Decode and encode new AAC. ALAC is not copied by this implementation. |
| Decodable audio → WAV | Decode to a larger 16-bit PCM working file. |
A cut or volume change rules out packet copy because it changes the timeline or sample values. Simply choosing M4A is therefore not a promise of lossless output.
Audacity: convert, or install FFmpeg?
Audacity documents different behavior by operating system. Its macOS build can import unprotected M4A (AAC) and MP4 files as shipped. On Windows and GNU/Linux, M4A (AAC) generally needs the optional FFmpeg library. Installing FFmpeg avoids making an MP3 just to get the file into Audacity. Conversion is the practical alternative when you cannot change that setup or the next device genuinely needs MP3. Protected store files are separate; changing the extension or format does not remove access controls.
When there is no waveform
Check that the original still plays in the application that created it. The browser may understand the M4A container but lack a decoder for the codec inside. MP3 and WAV then cannot run. A complete, unedited AAC track may still use the decoder-free M4A copy path; ALAC cannot. A damaged file or misleading extension needs to be re-exported from its original application. Keep the source until you have listened through the result.