MP4 to AU Converter

How to Convert MP4 to AU Online

1. Upload Your MP4 File: Drag and drop or click "Add Files" to select MP4 or M4V videos. Batch is supported — queue several clips and they convert sequentially.
2. Pick a Quality Preset: The default is Very High (Recommended). Choose Highest for archival, Medium for smaller files, or Low/Lowest for telephony-style 8-bit μ-law output. The preset drives the AU encoding (PCM bit depth or μ-law) used inside the container.
3. Set Audio Channel and Sample Rate (Optional): Audio Channel accepts Original, Mono, or Stereo. Sample Rate accepts Original, 8000 Hz, 12000 Hz, 16000 Hz, 24000 Hz, 44100 Hz, or 48000 Hz. For phone-system or javax.sound legacy output, set 8000 Hz + Mono.
4. Trim and Convert: Use Trim to set a start time and duration in seconds or HH:MM:SS.sss to grab one segment instead of the full track. Click Convert and download — files process in your browser session, no sign-up, no watermark.

Why Convert MP4 to AU?

MP4 is an ISO Base Media File Format container (ISO/IEC 14496-14) that almost always carries AAC audio alongside H.264 or H.265 video. AU is something very different: a 28-byte big-endian header (magic number .snd / 0x2e736e64) followed by raw audio samples, introduced by Sun Microsystems and adopted by NeXT in the late 1980s. Converting MP4 to AU strips the video track, decodes the AAC audio, and re-encodes the samples into AU's straightforward layout.

• Java javax.sound.sampled pipelines — The standard Java sound API reads AU natively without any plugin or third-party library; AAC inside MP4 needs an external decoder. AU was the format Sun shipped with the JDK and it remains the safest fallback for Java applets, JavaSound demos, and educational projects.
• Telephony and IVR prompts — Asterisk, FreeSWITCH, and traditional PBX gear consume 8 kHz mono μ-law as defined by ITU-T G.711. AU at 8000 Hz mono with the Lowest quality preset matches that profile and drops straight into hold music, voicemail greetings, or auto-attendant prompts.
• Legacy Solaris / SunOS / NeXTSTEP audio — Old workstations and the apps that ran on them (/dev/audio, NeXT Music Kit, early SoundEdit projects) expect AU. If you need to feed audio to vintage hardware emulators or restore a 1990s multimedia title, AU is the format the system asks for.
• Scientific computing and DSP teaching — MATLAB's audioread, GNU Octave, Python's scipy.io.wavfile siblings, and many CS-101 audio assignments still distribute reference clips as .au because the header is short enough to parse by hand. Stripping audio from a recorded lecture (MP4) into AU keeps the file readable in those toolchains.
• Embedded systems and microcontroller playback — μ-law AU is small, byte-aligned, and decodable in a few dozen lines of C, which is why hobby projects, retro arcade boards, and museum installations still use it.

MP4 vs AU — Format Comparison

AU Encoding Quick Guide

Frequently Asked Questions

Why is my AU file larger than the original MP4 even though there's no video?

MP4 audio is almost always AAC at roughly 96–192 kbps — a heavily compressed lossy codec. The Very High and Highest presets re-encode that audio as 16-bit or 24-bit PCM inside the AU container, which is uncompressed and runs around 1.4 Mbps for stereo 44.1 kHz. So a 60-second AAC track at ~1 MB becomes a ~10 MB PCM-in-AU file. Pick the Lowest preset with 8000 Hz mono if you want an AU that's smaller than the MP4 — that produces 64 kbps μ-law, about 480 KB per minute.

Should I pick μ-law or 16-bit PCM for AU output?

Pick μ-law (Lowest preset, 8000 Hz, Mono) if your target is a telephone system, an old Java applet, or any pipeline that expects "Sun audio" by default — μ-law was AU's original encoding and the most universally readable. Pick 16-bit PCM (Very High, 44100 Hz, Stereo) for music, podcasts, or anything where you'd otherwise reach for WAV. The header tells the decoder which one was used, so both are valid AU files.

What sample rate should I use for telephony or IVR systems?

8000 Hz mono. This is the rate defined in ITU-T G.711 (the μ-law / A-law standard) and the rate that Asterisk, FreeSWITCH, Twilio's <Play> verb, and almost every PBX expect for native playback. Higher rates either get downsampled by the phone system (wasted bandwidth) or rejected outright on legacy hardware.

Does the converter accept M4V files too?

Yes. M4V uses the same MPEG-4 Part 14 container as MP4 — the only practical difference is Apple's optional FairPlay DRM on iTunes purchases. Non-DRM M4V files (the kind you export from iMovie, Final Cut, or QuickTime) decode identically. DRM-protected M4V from the iTunes Store cannot be processed by any third-party converter.

Can I extract audio from one specific scene instead of the whole MP4?

Yes — use the Trim option. Set the start time and duration in seconds (30 and 15 for "15 seconds starting at the 30-second mark") or in HH:MM:SS.sss (00:01:30.250 for one minute thirty and a quarter). The converter seeks to that range, decodes only the requested audio samples, and writes them to the AU file with the chosen sample rate and channel layout.

Will the AU file play on Windows or macOS?

On macOS, QuickTime Player and the built-in afplay command both open AU. On Windows 11, the default Media Player no longer registers .au out of the box, so use VLC, foobar2000, or Audacity — all three play AU at any of its supported encodings. Web browsers do not play AU directly; if you need browser-playable audio, MP3 or AAC are better targets.

Why does AU use big-endian when most audio formats are little-endian?

Sun's SPARC and NeXT's Motorola 68k workstations were both big-endian processors, so the format was specified that way to match native CPU memory layout in the late 1980s. Modern x86 and ARM systems are little-endian, which means decoders byte-swap samples on read; this is handled transparently by every mainstream audio library, but it's why low-level C code reading AU often needs ntohl / __builtin_bswap32.

How does AU compare to WAV for the same use cases?

AU and WAV are both simple PCM containers. WAV (Microsoft RIFF, little-endian) dominates Windows and most modern audio software. AU (Sun, big-endian) dominates the Java and Unix worlds. They store the same audio data — the choice is about which decoder you're feeding. If you need WAV instead, see MP4 to WAV. For an MP3-compressed extract, see MP4 to MP3. For QuickTime sources, see MOV to AU.

Is AU still actively used in 2026?

It's a niche format today — most new projects pick WAV, FLAC, or MP3. But AU survives in three places: Java audio APIs (still shipped in the JDK), legacy telephony stacks that haven't been re-engineered, and academic / DSP courses that distribute reference clips. If a tool, textbook, or vendor specifies "AU file" or "Sun audio," that requirement isn't going away — and a reliable MP4 → AU path is the easiest way to feed modern recordings into those workflows.