The short answer
- M4A is an audio file format that stores audio in an MPEG-4 container, but its extension alone does not tell you the codec or whether the audio is lossless.
- For AAC-encoded M4A, Apple names Music and QuickTime Player as playback options. On a Mac, try opening the file in one of these applications. On Windows or a phone, choose an application whose documentation explicitly supports both M4A and the file's codec. Check the codec in the file's technical information to see whether it is AAC or ALAC, then check the application's supported-format list.
- An M4A extension does not guarantee safety. RFC 4337 describes malformed media streams that can overload a receiver or decoder and cause failures or crashes. It also permits MPEG-J in audio/mp4 content. This does not mean an ordinary AAC or ALAC M4A contains active content. Consider the source before opening an unexpected file.
M4A files: opening, codecs and conversion
M4A is an extension used for MPEG-4 audio. Apple identifies the file type as com.apple.m4a-audio. The container and audio codec are separate: M4A can contain AAC audio or lossless ALAC audio. The audio/mp4 MIME type describes MP4 content with audio and no visual presentation; it does not identify the audio codec.
Apple documents audio conversion in Music on a Mac. Preserve the original and choose an output supported by the receiving application. Converting to MP3 or another lossy format may discard audio data. Converting lossy M4A to WAV cannot restore discarded data or improve the original quality; use the original audio source when available.
How to open it
For AAC-encoded M4A, Apple names Music and QuickTime Player as playback options. On a Mac, try opening the file in one of these applications. On Windows or a phone, choose an application whose documentation explicitly supports both M4A and the file's codec. Check the codec in the file's technical information to see whether it is AAC or ALAC, then check the application's supported-format list.
For AAC-encoded M4A, Apple names Music and QuickTime Player as playback options. On a Mac, try opening the file in one of these applications. On Windows or a phone, choose an application whose documentation explicitly supports both M4A and the file's codec. Check the codec in the file's technical information to see whether it is AAC or ALAC, then check the application's supported-format list.
Apple documents audio conversion in Music on a Mac. Preserve the original and choose an output supported by the receiving application. Converting to MP3 or another lossy format may discard audio data. Converting lossy M4A to WAV cannot restore discarded data or improve the original quality; use the original audio source when available.
File safety
An M4A extension does not guarantee safety. RFC 4337 describes malformed media streams that can overload a receiver or decoder and cause failures or crashes. It also permits MPEG-J in audio/mp4 content. This does not mean an ordinary AAC or ALAC M4A contains active content. Consider the source before opening an unexpected file.
An unknown sender, a name ending in two extensions, or contents that do not match the extension — check before you open it.
Converting it
Apple documents audio conversion in Music on a Mac. Preserve the original and choose an output supported by the receiving application. Converting to MP3 or another lossy format may discard audio data. Converting lossy M4A to WAV cannot restore discarded data or improve the original quality; use the original audio source when available.
Convert a copy, never the original, then compare quality, metadata, and layout against what you started with.
Common questions
What is an M4A file used for?
M4A stores MPEG-4 audio. It can contain AAC or ALAC, so the filename extension does not establish whether the audio is lossy or lossless.
How do I open an M4A file for playback?
Use an application that supports the M4A container and its audio codec. Apple explicitly names Music and QuickTime Player for listening to AAC-encoded M4A files.
How do I open an M4A file on Windows?
Check the codec in the file's technical information to see whether it is AAC or ALAC. Check that a Windows player's supported-format list includes both M4A and that codec, then open the file from that player.
How do I open an M4A file on Mac?
For AAC-encoded M4A, try Music or QuickTime Player. Apple names these apps for AAC-encoded M4A; that does not mean every codec an M4A can hold will play.
How do I play an M4A file on a phone?
Check the codec in the file's technical information to see whether it is AAC or ALAC. Choose a phone application whose supported-format list includes both M4A and that codec, then use its file-opening function.
M4A versus AAC: which should I choose?
They describe different layers. Choose M4A when specifying the file container and AAC when specifying the audio encoding. An M4A file can contain AAC, so they are not mutually exclusive choices.
How can I tell whether an M4A file is lossless?
Check the codec in the file's technical information. ALAC is lossless; AAC is lossy. ALAC decoding reproduces the encoded uncompressed input bit for bit, but does not prove that the earlier source never underwent lossy compression.
Does converting M4A to MP3 reduce audio quality?
MP3 is a lossy output, so conversion may discard audio data. Keep the M4A original and, when possible, encode from the original audio source instead of repeatedly converting lossy copies.
Does converting M4A to WAV improve audio quality?
Converting a lossy M4A to an uncompressed WAV cannot recover discarded audio data or improve the original quality. Keep the original file.
What should I do if an M4A file will not open?
Check whether the player supports both M4A and the actual codec. For AAC M4A, try Apple's documented Music or QuickTime Player options. Failure to open alone does not establish corruption.
Is an M4A file received by email safe to open?
The extension alone cannot establish safety. RFC 4337 describes decoder failures from malformed streams and allows MPEG-J in audio/mp4. Consider the sender and file source; ordinary AAC or ALAC M4A is not thereby shown to contain active content.
Related formats
Sources
m4a — AVFileType · Apple 官方开发者文档
RFC 4337: MIME Type Registration for MPEG-4 · IETF 标准轨道 RFC
Apple Lossless Audio Codec · Apple ALAC 官方开源项目
Apple Digital Masters · Apple 官方技术文档
Convert a song to a different file format on your Mac with the Apple Music app · Apple 官方支持文档