The short version
If you do not want to think about it, pick MP3. It is the only format that every player, every car stereo, every phone and every editor made in the last twenty-five years will open. Everything below is about the cases where something better is available.
| Format | Opens nearly everywhere | Quality | Use it when |
|---|---|---|---|
| MP3 | Yes | Lossy | Compatibility matters more than anything else |
| M4A (AAC) | Modern devices, not older editors | Copied, no loss | You want the original audio untouched |
| WAV | Yes, on computers | Uncompressed | It is going into an editor |
| Opus in WebM | Browsers only | Lossy | Never, as an output — convert it away |
Why Audacity will not open your M4A
Audacity in a default installation cannot import M4A, MP4 or most video files. This is a licensing constraint rather than a technical one: AAC is patent-encumbered, so an open-source project cannot simply bundle a decoder. The usual advice is to install a separate library, but converting to MP3 or WAV first is faster and works every time.
The same applies to several free editors and to older versions of some paid ones. If software refuses your file with a vague error, the format is the first thing to check.
Phones
Android plays MP3, M4A, WAV, FLAC and Opus. iPhone plays MP3, M4A, WAV and, more recently, FLAC. Neither will accept a ringtone installed from a web page, which is a restriction of the phone rather than of the file.