MKV to AU Converter

MKV to AU Converter

This tool extracts the audio track from an MKV (Matroska) video and writes it into an .au file — the Sun Microsystems audio format (also seen as .snd) from the Unix-workstation and NeXT era. The video is discarded; only one selected sound track is kept. AU is a niche target today, so be clear on why you'd want it: a legacy Unix utility, an old Java program, or a retro-computing or academic pipeline that specifically reads the Sun audio format. If you just want the soundtrack to play or share normally, MKV to MP3 or MKV to WAV (the standard uncompressed-PCM choice) is almost always the better pick — AU offers no quality or size advantage over those.

MKV Format at a Glance

AU Format at a Glance

What This Converter Writes Into the AU

The .au most people picture is 8-bit mu-law at 8 kHz — the lossy companded format SunOS exposed through /dev/audio. This converter does not down-convert to that by default; it writes 16-bit big-endian linear PCM (encoding code 3) into the AU container, which is the muxer's default and a lossless encoding. What you get out therefore depends on what was inside the MKV:

• If the MKV's audio is AAC, AC-3, Opus, Vorbis, or MP3 (the common cases), it was already lossy-compressed. Wrapping it in PCM does not recover any detail that lossy encoding threw away — you simply get a larger, uncompressed file that sounds the same as the source track.
• If the MKV's audio is FLAC or another lossless track, the decode-then-PCM transfer keeps full fidelity: every sample the source carried ends up in the AU, with nothing discarded.

Either way, AU's linear PCM is uncompressed, so the .au is typically larger than the audio occupied inside the MKV. You are paying bytes for the container, not gaining fidelity.

How to Convert MKV to AU

1. Upload Your MKV File: Drag and drop your .mkv onto the page or click "+ Add Files". Several files can be queued and converted with the same settings.
2. Set Audio Sample Rate and Audio Channel: Leave Audio Sample Rate and Audio Channel on "Original" to copy the source track's native rate and layout untouched, or change them to resample or downmix to mono if a legacy target needs it.
3. Trim (Optional): Use the Trim control to export just a section of the audio, or leave it "Unchanged" to extract the whole track.
4. Convert and Download: Click "Convert" and download your AU file. No sign-up, no watermark.

Files are uploaded over an encrypted connection, processed on our servers, and deleted automatically a few hours after conversion — never shared or made public.

Frequently Asked Questions

Which audio track does it pull from an MKV with several streams?

An MKV can carry multiple audio tracks — an original language, dubs, and a director's commentary, for example. The converter extracts a single track (the file's default/first audio stream) and writes that into the .au. If you need a specific non-default language or commentary track and it isn't the one you get, that selection is beyond what a one-click AU export exposes; a tool that lets you choose the stream index, or a first pass through MKV to MP3 where you can isolate the track you want, is the way to handle it.

Will extracting MKV audio to AU improve the sound quality?

No. Converting cannot add detail that was never in the source. If your MKV carries AAC, AC-3, Opus, or MP3 audio — the usual case — that track was already lossy-compressed, and writing it into AU's linear PCM only makes a bigger file that sounds identical to the original. If the MKV's audio was FLAC or another lossless codec, the AU is a full-fidelity copy of it. In neither case do you regain quality the MKV didn't already hold.

Does the AU output use 8-bit mu-law like classic Unix .au files?

No. The historic .au people remember from Sun workstations was 8-bit mu-law at 8000 Hz, which is lossy and telephone-grade. This converter writes 16-bit big-endian linear PCM by default — the AU container's lossless default, encoding code 3 in the Sun spec — preserving full-bandwidth audio rather than companding it down to mu-law. If a specific legacy tool requires exactly 8-bit mu-law, this output will not match that encoding; you would need a tool that lets you force the mu-law codec.

Why is my .au file larger than the audio in the MKV?

Because AU's linear PCM is uncompressed. The audio inside an MKV is usually stored with a compressed codec like AAC, AC-3, or Opus, which is far smaller than raw PCM. When that track is written out as 16-bit PCM, every sample is stored in full, so the .au commonly grows several times larger than the compressed audio occupied in the MKV. The extra bytes are uncompressed data, not added quality.

Why would anyone convert to AU instead of MP3 or WAV?

Only for compatibility with something that specifically expects .au. Early Java was the classic case: its original sound API supported exactly one format — 8-bit mu-law, 8000 Hz, mono, Sun .au files — so applet and desktop audio of that era shipped as AU. Other realistic cases are a legacy Unix or NeXT-lineage program that reads the Sun format natively, or an academic or retro-computing pipeline that documents .au as its interchange format. For listening, sharing, or editing, MKV to MP3 or MKV to WAV is the better choice.

Is the AU output big-endian, and does that matter?

Yes. The AU format stores its header and sample data in network (big-endian) byte order regardless of the host machine, and the 16-bit PCM this converter writes follows that. It matters only if a downstream tool reads the raw bytes assuming little-endian — well-behaved players honor the header and handle it correctly. WAV, by contrast, is little-endian, which is one of the technical differences between the two containers even though both can carry the same PCM samples.

How are my files handled, and how long do you keep them?

Files are uploaded over an encrypted connection, processed on our servers, and deleted automatically a few hours after conversion — never shared, never made public, with no sign-up and no watermark. In our testing, a one-minute MKV with a stereo 16-bit/44.1 kHz track produced an .au of roughly 10 MB, since CD-quality stereo PCM runs about 10 MB per minute regardless of how compact the audio was inside the original MKV.