Add your AU file
Drop one or more .au files onto the converter above, or browse for them. They load into browser memory only — nothing is uploaded, so there is no size-based pricing and no server queue.
AU to M4A
Re-encoding .au audio as .m4a changes how the sound is compressed and which players will accept it. Free, private, and validated — the file never leaves your browser.
100% private for supported formats
Runs on your deviceConvert supported files
Drop AU files here
Batch files can each use a different output. Nothing uploads for local conversions.
Working inputs include camera RAW, browser-local audio/video, PDF, CBZ/CBR comics, office documents, ebooks, markup, 3D models, structured text, images, and archives.About converting Sun audio to MPEG-4 audio
Re-encoding .au audio as .m4a changes how the sound is compressed and which players will accept it. An .au file is the Sun audio format, one of the earliest structured audio files, created for Unix workstations. A minimal header followed by sample data, classically 8 kHz mu-law telephony audio, makes .au about as simple as an audio format can get.
An .m4a file is an MPEG-4 container that holds only audio, most often AAC and occasionally Apple Lossless. Apple coined the .m4a extension so audio would not be confused with .mp4 video, and it remains the default for iTunes purchases, Apple Music downloads, and Voice Memos. For this route the practical draw is real tagging and artwork support, unlike bare .aac streams and gapless playback metadata for continuous albums — balanced against the extension hides whether the contents are lossy or lossless, muddying quality expectations, which is worth knowing before you commit a large batch.
Why convert AU to M4A?
The practical trigger for this conversion is usually a mismatch: with .au, essentially obsolete outside legacy systems. Switching to .m4a buys you real tagging and artwork support, unlike bare .aac streams, which is why it is the better fit for managing music inside iTunes and Apple Music libraries. Because the conversion runs locally, trying it costs nothing but a few seconds of compute on your own machine.
AU vs M4A at a glance
| Aspect | Sun audio (.au) | MPEG-4 audio (.m4a) |
|---|---|---|
| Format type | Lossless — every pixel or sample is preserved exactly | Lossy — discards detail it judges invisible to shrink files |
| How it stores data | a 24-byte header carries a '.snd' magic number, data offset, encoding type, sample rate, and channel count | an ISO base media container restricted to audio tracks, structurally a sibling of MP4 |
| Strongest at | maintaining sound assets in vintage Unix and Java software | managing music inside iTunes and Apple Music libraries |
| Weak spot | essentially obsolete outside legacy systems | the extension hides whether the contents are lossy or lossless, muddying quality expectations |
| Metadata | an optional annotation field in the header can hold a short text string; no formal tag structure exists | iTunes-style metadata atoms: title, album, artwork, composer, BPM, and the iTunSMPB gapless-playback data |
Pick M4A as the output
Select .m4a in the output menu next to each file. The menu only offers targets this engine can genuinely produce, so if M4A is selectable, the route is real and validated.
Convert on your device
Press Convert. The transcode runs on an FFmpeg engine compiled to WebAssembly, so the audio never leaves your device.
Verify and download
Each result is signature-checked before the download unlocks, so a failed encode can never masquerade as a valid M4A file. Outputs keep the original filename with the .m4a extension.
AU to M4A questions, answered
Will converting AU to M4A lose quality?
Some. Since au is lossless-oriented and m4a is lossy-oriented, the encoder will discard detail it judges least visible; the quality slider controls how aggressive that is. Keep the original .au if you may need a pristine master later.
Is it private to convert AU files here?
Yes — the .au file is processed inside your browser tab and never uploaded. The transcode runs on an FFmpeg engine compiled to WebAssembly, so the audio never leaves your device. Close the tab and the file is gone from memory.
Where will the M4A file work?
Browsers, Windows, Android, and every Apple device play AAC-based .m4a files without extra software. The ALAC-filled variety is slightly narrower, since some inexpensive devices decode only the AAC form.
Will the M4A file be bigger or smaller than my AU?
Smaller, usually dramatically so — that is the main reason this route exists. The quality slider trades size against fidelity; the default sits at the point where most people cannot spot the difference.
What happens to the metadata in my AU file?
This converter strips container metadata deliberately (the FFmpeg command runs with metadata mapping disabled) so exported files do not leak recording details. In the source, an optional annotation field in the header can hold a short text string; no formal tag structure exists; none of it is transmitted anywhere because the file never leaves your browser.