FLV to AU Converter

Extract Audio from FLV to AU: What This Tutorial Covers

FLV is Adobe's Flash Video container — a relic of the YouTube-and-Flash era whose picture few modern players still touch, though VLC and ffmpeg still open the files. This tutorial is for anyone who needs the soundtrack out of an old FLV and specifically wants it as an AU file: a Sun Microsystems audio format from the late 1980s that Java's AudioClip and older Unix audio pipelines read natively. You will end up with an audio-only .au file — the video is discarded — and you will understand why that file is bigger than the audio inside the FLV but no higher in fidelity.

How to Convert FLV to AU

1. Upload Your FLV File: Drag and drop or click "Add Files" to select FLV videos from your computer. Batch is supported — queue several clips and they convert one after another.
2. Pick a Quality Preset: The default Very High (Recommended) writes 16-bit big-endian PCM, AU's standard linear encoding. Choose Highest for the largest, most faithful PCM, or Lowest to fall back to AU's original 8-bit μ-law telephony encoding for the smallest file.
3. Set Audio Sample Rate and Channel (Optional): Audio Sample Rate offers Original, 8000, 12000, 16000, 24000, 44100, or 48000 Hz; Audio Channel offers Original, Mono, or Stereo. For Java AudioClip or phone-system playback, set 8000 Hz and Mono.
4. Trim and Convert: Use Trim to set a start time and duration if you only need one segment, then click Convert. No sign-up, no watermark.

Walk-through: Choosing the Right AU Encoding

AU is unusual among audio formats because it long predates the little-endian x86 world — Sun's SPARC and NeXT's Motorola 68k workstations were big-endian, so AU stores its samples big-endian, and that is still the encoding this converter writes by default. The 24-byte header (six 32-bit words) begins with the signature bytes 2e 73 6e 64 — the ASCII .snd that gives the format its alternate .snd extension. Within that header AU records which encoding it used, so the file stays a valid .au whichever preset you choose:

• Want general music or a faithful copy? Leave Quality Preset at Very High (Recommended) — 16-bit big-endian PCM, the safest broadly readable AU encoding.
• Want maximum compatibility with old systems? Pick Lowest. That writes 8-bit μ-law, AU's original telephony encoding and the one most legacy "Sun audio" tools expect.
• Feeding a Java AudioClip or phone prompt? Combine Lowest (μ-law) with 8000 Hz and Mono — that is the classic 8 kHz Unix system-sound profile.
• Archiving? Highest keeps the most PCM detail the source can supply, at the cost of a larger file.

One honesty note that matters here: an FLV's audio track is almost always MP3 or AAC, both of which are lossy — perceptual detail was thrown away when the FLV was first encoded. Decoding that into AU's uncompressed PCM makes the file larger, but it cannot put back what the original lossy codec discarded. You get a faithful, bulky copy of already-lossy audio, not new fidelity.

Common Errors and How to Fix Them

• "Output is silent" — The FLV may be video-only, or its audio track uses a codec the decoder skipped. Confirm the source actually has sound by playing it in VLC first.
• "AU file is huge for a short clip" — Expected with the default PCM preset: uncompressed 16-bit stereo at 44.1 kHz runs roughly 10 MB per minute. Switch to the Lowest preset at 8000 Hz Mono for a much smaller μ-law file.
• "My player won't open the .au" — Many modern media players dropped AU support. VLC, Audacity, and Java-based tools still read it; if you only need playback, convert FLV to MP3 instead.
• "Upload is slow or times out" — Large FLV archives upload before they process; the limiting factor is upload time, not the conversion itself. Trim to the segment you need before uploading if possible.
• "The picture is gone" — That is intended. This tool extracts audio only and discards the video stream.

When This Doesn't Work — and What to Use Instead

If your goal is a small, broadly playable file to share or keep on a phone, AU is the wrong target — it is a niche legacy format, honestly useful today only for Java AudioClip resources, older Unix audio pipelines, telephony prompts, or coursework that distributes reference clips as .au. For everything else, convert FLV to MP3 gives a far smaller, universally playable file; because most FLVs already carry MP3 audio, that path is often close to a clean copy rather than a re-encode. Corrupted or partially downloaded FLVs (a common fate for old streamed Flash content) may decode with glitches or stop early — there is no way to recover audio that was never fully written. If you specifically need the old AU/Sun heritage but for a different source, convert M4V to AU follows the same approach.

Frequently Asked Questions

Why is my AU file bigger than the FLV's audio if the video was removed?

Because the FLV's audio is MP3 or AAC — lossy, compressed formats at roughly 96–192 kbps — while the default AU output is uncompressed 16-bit PCM at about 1.4 Mbps for stereo 44.1 kHz. So a one-minute MP3 track inside an FLV decodes into a roughly 10 MB PCM-in-AU file. Those extra bytes are not extra fidelity; they are an uncompressed copy of audio that already lost detail during MP3 or AAC encoding. Choose the Lowest preset at 8000 Hz Mono for an AU that is actually smaller (8-bit μ-law, about 480 KB per minute).

Which encoding does the AU output use — μ-law or PCM?

By default, 16-bit big-endian linear PCM, which matches AU's big-endian heritage and suits music or general audio. The Lowest quality preset switches to 8-bit μ-law at 8000 Hz — AU's original encoding and the most universally readable by legacy tools, ideal for Java applets, telephony prompts, or any system that expects classic "Sun audio." The AU header records which encoding was used, so both produce valid .au files.

Can converting an FLV to AU improve the audio quality?

No. The FLV's audio was already encoded with a lossy codec (MP3 or AAC), so the detail discarded at that stage is gone for good. Decoding it into AU's uncompressed PCM gives you a larger, faithful copy of the lossy audio, but it cannot reconstruct anything the original throw-away step removed. If you want the best possible quality, start from the original master, not an FLV.

Why does AU use big-endian byte order when modern formats are little-endian?

Sun's SPARC and NeXT's Motorola 68k workstations were big-endian processors, so when the format was specified in the late 1980s the samples were stored big-endian to match native CPU memory layout. Modern x86 and ARM machines are little-endian, so their audio libraries byte-swap AU samples transparently on read. It is only a concern in low-level code that parses the 24-byte header by hand. In our testing, a one-minute stereo FLV soundtrack decoded to AU at the default Very High preset produced a roughly 10 MB 16-bit PCM .au, while the Lowest μ-law preset at 8000 Hz Mono produced about 480 KB.

Is AU still worth using in 2026, or should I pick a different format?

For a new project, pick MP3, WAV, or FLAC — AU is niche today. But it survives where it is specifically required: Java audio APIs still ship .au support in the JDK, older telephony stacks expect 8 kHz μ-law, and many DSP or computer-science courses distribute reference clips as .au because the short 24-byte header is easy to parse by hand. If a tool, textbook, or vendor explicitly asks for "AU" or "Sun audio," this conversion feeds a modern FLV recording into it. If you only need a small, broadly playable file, convert FLV to MP3 is the better target.

How are my files handled, and how long are they kept?

Files are uploaded over an encrypted connection and processed on our servers, not in your browser. They are deleted automatically a few hours after conversion and are never shared or made public. There is no sign-up and no watermark on the output. The practical limit on large FLV archives is upload time, since the decode and re-encode run on our servers rather than on your device.