AU to M4B converter
A free AU to M4B converter for Sun Audio files. Browser-based pairs here usually run in this tab, with no file upload.
Drop your AU files here
or browse your device · paste with Ctrl+V
Accepts .au, .snd files. You can queue several files; each is processed one after another on your own device.
Your files never leave this device — there is no upload and no server to delete them from.
- 01
Add your AU files
Drop them onto the box above or click to browse. They are read straight from disk — nothing is sent anywhere.
- 02
Check the settings
M4B is already selected. Open the gear icon on any row if you want to change quality, dimensions or codec before converting.
- 03
Download your M4B
Press convert and wait for the row to finish. Convert several files and you can take them all as a single ZIP.
AU
Sun AudioThe Unix system-sound format from Sun Microsystems, usually holding 8-bit mu-law audio. Historically important, practically obsolete.
- Developed by
- Sun Microsystems
- Released
- 1992
- Extension
- .au, .snd
- MIME type
- audio/basic
- Compression
- Lossy or lossless
M4B
MPEG-4 AudiobookAn M4A that flags itself as an audiobook, so players remember your position and honour chapter markers. Audible titles land here once stripped of DRM.
- Developed by
- Apple
- Released
- 2003
- Extension
- .m4b
- MIME type
- audio/mp4
- Compression
- Lossy
- M4B is lossy, so if the source AU was lossless this is the first lossy step. If it was already lossy, any additional impact is often smaller.
Questions about converting AU to M4B
Is this AU to M4B converter free?
Yes. Converting AU to M4B here is free, with no account, no watermark and no daily quota. The work runs on your own device, so there is no server cost to pass on to you.
Is my AU file uploaded anywhere?
For browser-based pairs, your file is read locally and converted by ffmpeg in this tab. No file payload is uploaded; you may still see analytics and engine downloads in network logs.
Is there a file-size limit for AU to M4B?
There is no imposed upload cap when it runs in-browser, but each conversion still depends on your device and browser memory. Large videos are the first practical limit, then memory-heavy documents or images.
Does converting AU to M4B lose quality?
M4B is a lossy format, so some detail is discarded when it is encoded. At the default quality setting the change is often not visually obvious; keep your original AU file if you might need it again.
Can I convert several AU files to M4B at once?
Yes. Drop in several files and each gets its own row; they are processed one after another on your device. You can download them one by one or as one ZIP.
Does AU to M4B work on a phone, Mac or Chromebook?
Most modern browsers on Windows, macOS, Linux, ChromeOS, Android and iOS work. Very large videos are often more reliable on desktop or a browser with more memory.
Why do some conversions take longer on first run?
The conversion engine has to download first. Small codec bundles are fast, while ffmpeg is commonly 12–30 MB; later conversions of similar type use the cached engine.
I converted a file and the output is blank or unusable. What should I check?
Start over with default settings, then change one option at a time (quality, bitrate, page size, or codec) to isolate what changed behavior. If it keeps failing, retry with a smaller file and confirm the source format matches the pair.
Convert other formats to M4B
41 formats can become M4B here.
Convert AU to other formats
A AU file can become any of these.
M4B to AU
The reverse conversion
MP4 to M4B
MP3 to M4B
WAV to M4B
Extract Audio from Video
Pull the soundtrack out of a video as an MP3.…
Compress Audio
Lower the bitrate of audio files to save space.…
Trim Audio
Cut a section out of an audio file.…
Normalise Audio
Even out loudness across files so nothing is jarring…