AU to MP4 Converter
Processing entirely on your device — large files may take a moment.
Sun Microsystems Audio and MPEG-4 Video exist for different jobs: AU is the µ-law audio format of old Unix systems and Java applets — tiny mono files from another era. Convert to MP4 when a video must play everywhere — it is the one format you never have to worry about.
Because the whole conversion happens on your device, it is instant, unlimited and genuinely private — no accounts, no watermarks, no files on anyone's server. Multiple files are converted in one go.
How AU compares to MP4
| AU | MP4 | |
|---|---|---|
| Compression | µ-law or PCM | H.264 / HEVC / AV1 + AAC |
| Quality | Mono voice-grade (µ-law) | Depends on encode settings |
| Compatibility | Legacy Unix tools | Plays everywhere |
| Best for | Old system sounds and recordings | Distribution, web, storage |
How to convert AU to MP4
- 1
Select your files
Select your AU audio file.
- 2
Convert instantly
The engine draws the waveform and syncs it as a video frame to the original audio.
- 3
Download results
Download the MP4 — upload-ready for YouTube or Bilibili.
About the AU format
AU is Sun Microsystems' audio format from the Unix workstation age: a small header followed by µ-law (or PCM) samples. Java's applet era shipped it everywhere, and old Unix sound tooling still emits it.
Modern players increasingly skip it. Converting AU to WAV preserves the samples bit-for-bit while moving the file into universal territory — the standard rescue path for system sounds and old recordings.
+ Strengths
- ✓Simple, lossless sample storage
- ✓Native to old Unix/Java tooling
! Limitations
- −µ-law flavor sounds dated and mono
- −Dropped by modern players
About the MP4 format
MP4 (MPEG-4 Part 14) is the universal delivery container of digital video. It wraps H.264, HEVC or AV1 video with AAC audio, and its structure — separate metadata (moov) and media atoms — is what makes streaming, seeking and fast-start playback possible. Phones, cameras, TVs, browsers and every social platform assume MP4 first.
Quality is entirely a function of the codec and bitrate chosen at encode time, so an MP4 can be a 10 Mbps HEVC master or a postage stamp. That flexibility plus total playback coverage is why nothing has displaced it in twenty years — conversion between formats almost always ends in MP4.
+ Strengths
- ✓Plays on every device and platform in existence
- ✓Streaming-ready structure (fast start, seeking)
- ✓Choice of modern codecs: H.264, HEVC, AV1
! Limitations
- −Quality depends entirely on encode settings
- −HEVC variants hit licensing gaps on some platforms
Why convert AU to MP4?
Think of it as moving your file to the container that fits the job. µ-law flavor sounds dated and mono MP4 works differently: Plays on every device and platform in existence If any of that sounds like your task, run the conversion — it takes seconds and nothing leaves your device.
Frequently asked questions
Is this AU to MP4 converter really free?
Yes — completely free, with no sign-up, no watermark and no daily conversion limits. The site is supported by advertising.
Are my files uploaded to a server?
No. The conversion runs entirely inside your browser using WebAssembly, so your files never leave your device — that is what makes it instant and private.
Does it work on Windows, Mac, iPhone and Android?
Yes. Anything with a modern browser works: Windows, macOS, Linux, Android and iOS. No software installation is required.
Is there a file size limit for AU to MP4?
The only limit is your device's memory, since nothing is uploaded. Files up to a few hundred megabytes work fine on a modern desktop browser; very large files simply take a little longer.
Can I convert a whole folder of AU files?
You can select many files at once and they are processed in a single batch. Output files land in one ZIP download when there is more than one result.
Are tags like title and artist preserved?
Tag handling varies between target formats and this conversion focuses on the audio itself. For archival copies where tags matter, keep the original file; the converted copy is for playback and sharing.