AU to MP3 Converter

How to Convert AU to MP3 Online

1. Upload Your AU File: Drag and drop or click "Add Files" to select .au or .snd files. Sun/NeXT workstation recordings, classic Java applet sound assets (Java 1.0/1.1 shipped .au), Solaris and SPARCstation desktop sounds, and old academic speech corpora all work. Batch is supported — convert an entire archive folder in one pass.
2. Pick the MP3 Bitrate Mode: Default is the MP3 codec at 128 kbps constant bitrate. Choose 96 kbps for the smallest files (fine for 8 kHz μ-law speech archives), 128–160 kbps for typical voice and casual listening, 192–256 kbps for music and clean PCM-encoded AU sources, or 320 kbps for the highest MP3 quality. Variable bitrate (VBR) presets are also available when you want quality-driven sizing instead of a fixed kbps.
3. Set Sample Rate, Channels, and Trim (Optional): Pick 8 kHz to keep the AU's native rate (typical for legacy μ-law speech), 22.05 / 24 / 32 kHz for FM-radio-tier output, or 44.1 / 48 kHz for CD/video standards. Switch between mono and stereo via the channel selector. Optionally trim with start time + duration in HH:MM:SS.sss format to extract a single phrase or sample from a longer .au capture.
4. Convert and Download: Click Convert. Files convert in your browser session and download individually or as a ZIP — no sign-up, no watermark.

Why Convert AU to MP3?

AU (the Sun/NeXT audio format, with extension .au or .snd) is a legacy container introduced by Sun Microsystems in the late 1980s for Unix workstations and adopted as the native sound format of early Java — the original sun.audio API only played .au. It's a small header followed by raw audio samples, usually 8-bit μ-law at 8 kHz mono, sometimes 16-bit linear PCM. MP3 is the universally supported lossy audio format that plays everywhere: every browser, every phone, every car stereo, every smart speaker. Converting AU → MP3 trades a tiny amount of fidelity (often imperceptible at the source's 8 kHz speech bandwidth) for files that are dramatically smaller and play without third-party tooling.

• Legacy Java audio assets — Java 1.0/1.1 applets used sun.audio.AudioPlayer and only knew .au. Modernizing those sounds for current web pages, Android MediaPlayer, JavaFX, or Unity means MP3 (or WAV) so any HTML5 <audio> tag plays them out of the box.
• Solaris and SPARCstation archives — Decades of corporate voicemail, university lab recordings, and SunOS desktop sounds were captured as .au at 8 kHz μ-law. MP3 at 64–96 kbps preserves the entire intelligibility of speech archives at roughly half the size of the original .au and plays on any device.
• Listening copies of scientific corpora — Older bioacoustics datasets (birdsong, marine biology), seismic monitoring outputs, and 1990s-era research collections were published in .au. Converting to MP3 produces small, shareable listening copies for collaborators while you keep the original .au or a WAV master for analysis in MATLAB, Python, or Praat.
• Email, Discord, Slack, and WhatsApp — Modern messaging platforms display .au as an unknown attachment with no inline preview. MP3 plays inline everywhere, fits easily under Discord's 10 MB free-tier limit, and is the format voice-note features expect.
• Podcast and radio re-broadcast — Broadcast automation systems, podcast hosts, and radio playout software accept MP3 universally; AU is rejected outright. A 1-minute 8 kHz μ-law .au at ~480 KB becomes a 1 MB MP3 at 128 kbps that's instantly ingestible.
• iPod, car stereo, and old MP3 player support — MP3 is the lowest common denominator for hardware playback going back two decades. AU is not on any consumer device's compatibility list.

AU vs MP3 — Format Comparison

MP3 Bitrate Choice for AU Source

Frequently Asked Questions

Will converting AU to MP3 improve audio quality?

No — encoding can never add information that isn't in the source. If your AU is 8 kHz 8-bit μ-law (Sun's classic default), the resulting MP3 has at best the same intelligibility, and at worst a tiny bit less because MP3 is lossy. The reason to convert is compatibility, not quality. If your AU happens to contain 16-bit linear PCM at 44.1 kHz (rarer but possible), encoding to MP3 at 192 kbps or higher preserves nearly all perceived quality at a fraction of the size.

What MP3 bitrate should I use for an 8 kHz μ-law AU file?

96 kbps mono is plenty. The source's 8 kHz sample rate caps usable audio bandwidth at ~4 kHz (the Nyquist limit), which is well within what 64–96 kbps MP3 can encode transparently for speech. Anything above 128 kbps for an 8 kHz μ-law AU is wasted bytes — the MP3 cannot encode detail that doesn't exist in the source.

Does the .snd extension also convert?

Yes for Sun/NeXT .snd files — they use the same container as .au and are handled identically by XConvert's pipeline. Note that classic Mac OS System 7 also used .snd for an unrelated sound resource format from the pre-OS-X era; those rare files need a Mac-specific extractor first and won't decode here.

Should I convert to MP3 or to WAV?

Pick MP3 when you want small files, broad device support, or a listening copy. Pick WAV when you want a lossless archival master, when you'll edit the audio in a DAW, or when downstream telephony tooling needs μ-law preserved (WAV can wrap μ-law; MP3 cannot). A common pattern is to keep the original .au plus a WAV master for editing, and produce MP3s for distribution.

Why does my Java 1.0 applet sound bigger as MP3 than as the original AU?

Because the original was likely 8-bit μ-law at 8 kHz mono — about 8 KB per second — while MP3's minimum useful bitrate is around 32 kbps (~4 KB per second), and most encoders default to 128 kbps stereo, which is ~16 KB per second. To get a smaller MP3 than the source, choose 64–96 kbps mono explicitly. Even then, MP3 has a ~400-byte frame header overhead and a fixed minimum bitrate floor that μ-law doesn't, so for very short clips the AU may stay smaller — but compatibility goes from "almost nowhere" to "everywhere."

Can I batch-convert a directory of legacy .au files to MP3?

Yes. Drop the entire folder into the upload area and every file converts in parallel within your browser session. Settings apply uniformly, which is ideal when migrating an archive of recordings made on the same workstation. Output downloads individually or as a single ZIP. For larger collections, processing happens locally in your browser session — there are no per-file rate limits beyond your machine's CPU.

Will MP3 metadata (title, artist) survive from the AU file?

AU has no tagging system — there is no title, artist, or album field in the Sun/NeXT container, only an optional 24-byte annotation field that most files leave blank. So converting AU → MP3 cannot carry tags forward (there are none to carry). After conversion, you can add ID3v2 tags to the MP3 in any tag editor (Mp3tag, Kid3, MusicBrainz Picard, iTunes/Music) or directly in your file manager.

What if my AU file won't open in Audacity or VLC?

A small fraction of .au files in old archives are actually mislabeled — they're either headerless raw PCM, NeXT .snd Mac System 7 resources, or files truncated by FTP transfer in ASCII mode (a classic 1990s bug). XConvert reads the standard Sun/NeXT magic bytes (.snd, hex 2E 73 6E 64) and decodes μ-law, A-law, and 8/16/24/32-bit PCM AU variants. If a file fails, inspect the first four bytes — anything other than 2E 73 6E 64 means the file isn't a real AU and needs format identification first.