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 — the lossy companded format SunOS exposed through /dev/audio — and that is in fact what this converter writes by default: 8-bit mu-law (encoding code 1), at the source's sample rate and channel layout unless you change them. 16-bit big-endian linear PCM (encoding code 3, lossless) or A-law can be selected in the advanced options. 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. Re-encoding does not recover any detail that lossy encoding threw away — and the mu-law default is telephony-grade companding, so the AU can only sound the same or slightly worse than the source track.
• If the MKV's audio is FLAC or another lossless track, select 16-bit PCM (PCM_S16BE) in the advanced options to keep full fidelity; the mu-law default packs roughly 14 bits of dynamic range into 8 and is not a lossless copy.

Either way, AU stores samples raw — one byte per sample per channel with mu-law, two with 16-bit PCM — so the .au is typically larger than the compressed 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 only makes a bigger file that sounds no better than the original — the default mu-law encoding is telephony-grade and slightly reduces fidelity. If the MKV's audio was FLAC or another lossless codec, the AU is a full-fidelity copy only if you select 16-bit PCM (PCM_S16BE) in the advanced options. 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?

Yes. The historic .au people remember from Sun workstations was 8-bit mu-law, and that is what this converter writes by default — encoding code 1 in the Sun spec — though at your chosen sample rate and channel count rather than being locked to 8000 Hz mono. Mu-law is lossy and telephone-grade; if you'd rather keep full-bandwidth linear audio in the container, select 16-bit big-endian linear PCM (encoding code 3, PCM_S16BE) in the advanced options. A-law is available there too.

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

Because AU stores every sample raw rather than perceptually compressed. The audio inside an MKV is usually stored with a compressed codec like AAC, AC-3, or Opus, which is far smaller than raw samples. The default 8-bit mu-law output stores one byte per sample per channel — about 5.3 MB per minute at 44.1 kHz stereo — and 16-bit PCM doubles that, so the .au commonly grows several times larger than the compressed audio occupied in the MKV. The extra bytes are raw 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 any multi-byte sample data in network (big-endian) byte order regardless of the host machine; the optional 16-bit PCM output follows that, while the default 8-bit mu-law samples are single bytes, where order doesn't arise. 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.

Does the AU keep the MKV's original sample rate and channels?

Yes, by default. Both "Audio Sample Rate" and "Audio Channel" start on "Original," so the extracted track is written into the .au at the same rate (44.1 kHz, 48 kHz, and so on) and the same channel layout — stereo, mono, or surround — that the MKV carried. Change those controls only if a legacy target needs a specific rate or a mono downmix; otherwise the AU keeps the source track's original sample rate and channel layout, decoded and written out as 8-bit mu-law (or 16-bit linear PCM if you select it in the advanced options).

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. A one-minute MKV with a stereo 44.1 kHz track produces an .au of roughly 5.3 MB with the default mu-law encoding (one byte per sample per channel), or about double that if you select 16-bit PCM — regardless of how compact the audio was inside the original MKV.