SOUND FLIPPER GUIDES
M4A, AAC, ALAC and Opus: inspect what is inside
An M4A filename can contain AAC, ALAC or Opus. Learn how inspection changes the right conversion choice and what the preview supports.
By Singularity Shift Ltd · Updated · Preview guide
The extension does not answer the quality question
M4A is normally an audio-oriented MPEG-4 container. AAC, ALAC and Opus are codecs that can live inside it. Files with the same .m4a ending may therefore use different compression and require different preservation rules.
AAC and Opus are lossy. ALAC, Apple Lossless, can preserve supported integer PCM samples. Apple publishes an ALAC reference implementation. That does not mean every file labelled M4A is lossless, or that every ALAC representation is qualified by this app.
Use the inspection result
Select the file and read the detected codec, rate and channel count before choosing an output. Sound Flipper's qualified M4A reader admits AAC-LC, 16/24-bit integer ALAC and mapping-family-0 mono/stereo Opus. A tested music-export layout with one timed subtitle track and recognized C2PA provenance is supported; the app warns that these extras are omitted during conversion. It refuses encryption, video, ambiguous tracks, unsupported channel layouts and precision it cannot preserve.
For an AAC or Opus source, WAV means storing the decoded float samples; missing detail is not restored. For a supported ALAC source, WAV can hold the original integer samples exactly. The same WAV button has a different quality implication because the inputs differ.
A simple decision example
Suppose a music editor requires WAV and inspection shows ALAC, 24-bit, 48 kHz, stereo. A matching 24-bit WAV is a preservation path. If inspection instead shows AAC-LC, the app decodes to float32 WAV. That avoids a forced integer reduction, but the recording still has AAC's earlier information loss.
If a destination requires MP3, both sources need MP3 encoding. Starting from ALAC avoids an earlier lossy generation; starting from AAC adds another. When the destination already accepts your M4A, retaining the original may be the better choice.
Duration is more than a filename
AAC can involve encoder delay and padding. M4A timing information can describe which samples belong to the presented recording. Our tests compare that declared presentation with native decoding, including awkward lengths. We do not compare only the duration of a source used before encoding and assume the container stored it perfectly.
Raw AAC/ADTS is different: its admitted form does not declare an original gapless length, so decoded priming and padding remain and a warning explains that limitation. Do not assume that M4A and a file ending .aac have identical timing.
Useful next steps
Use M4A to WAV for editing or M4A to MP3 for a specific compatibility need. Review metadata separately: audio preservation does not guarantee that every tag or artwork field survives. See the metadata guide.