AU to M4A Converter

AU vs M4A — Which Should You Convert To?

.au is the Sun Microsystems / NeXT audio format from the Unix-workstation and early-Java era — modern phones and music apps often refuse to open it, or misread the .au extension entirely. Converting to M4A (AAC in an MPEG-4 container) gives you the format Apple devices, Android, and every modern player treat as a first-class default. One honest caveat decides what you get: if your AU holds telephone-companded μ-law/A-law audio, the M4A plays everywhere but sounds like the source; if it holds linear PCM, you get a clean, small AAC copy of the full-quality audio.

AU vs M4A — Format Comparison

When to Pick M4A

• You want the file to play on an iPhone, iPad, or in Apple Music / iTunes without a second thought — M4A/AAC is Apple's native audio default.
• You need broad device support with a small file size: AAC is supported in Chrome, Edge, Safari 4+, and Firefox 22+ (via the OS decoder), and on essentially every modern phone.
• Your AU holds linear PCM and you want a compact, taggable copy rather than a bulky lossless one.

When to Stay on (or Convert AU Elsewhere)

• You need a bit-exact lossless archive of a linear-PCM AU — use AU to FLAC instead, which preserves every sample and adds tags.
• You want the single most universally playable lossy format across old and new software — AU to MP3 is the safe default.
• You're loading the audio into an editor (Audacity, Audition) that prefers uncompressed input — convert to AU to WAV for a clean PCM working copy.

How to Convert AU to M4A

1. Upload Your AU File: Drag and drop or click "+ Add Files" to select .au or .snd recordings — Sun/SPARCstation desktop sounds, Java 1.0/1.1 applet audio, and old Unix speech clips all work. Batch upload converts a whole folder at once.
2. Choose Your Quality Preset: Leave Quality Preset on "Very High (Recommended)" for a clean AAC encode, or pick a lower preset to shrink the file. For an exact target, switch to Custom Bitrate (e.g. 128 kbps) or Constant Bitrate and choose a value.
3. Adjust Sample Rate, Channel, or Trim (Optional): Leave Audio Sample Rate and Audio Channel on "Original" to copy the AU's native rate and layout, or change them to resample/remix. Use Trim to keep only a section of a longer recording.
4. Convert and Download: Click Convert. Files are processed on our servers and download individually or as a ZIP — no sign-up, no watermark.

Frequently Asked Questions

Will converting my AU to M4A improve the sound quality?

No — and the reason depends on what's inside the AU. AAC is a lossy codec, so it can shrink and modernize your audio but cannot add detail that was never recorded. If your AU holds 8-bit μ-law or A-law (the classic Sun telephone-companded default), the M4A will sound like the source — the same narrow ~8 kHz, telephone-grade fidelity — just in a container modern phones actually play. If your AU holds linear PCM (16/24/32-bit), the M4A is a clean single-generation AAC encode of that full-quality audio at a much smaller size. Either way you keep what you had; you don't regain quality the original capture never had.

How can I tell whether my AU is μ-law or linear PCM before converting?

Open the file in a tool that reports the codec — VLC's Tools → Codec Information, the free MediaInfo app, or ffprobe file.au. A codec line reading "pcm_mulaw" or "pcm_alaw" means it is 8-bit companded; "pcm_s16be" or higher means linear PCM, where the AAC encode preserves genuinely full-quality audio. A sample rate of 8000 Hz with one channel is the tell-tale signature of a classic Sun μ-law recording — fine to convert, just don't expect hi-fi from it.

Why won't my .au file play on my phone or in modern music apps?

The AU format predates almost every consumer device on the market — it was a Unix-workstation and early-Java standard, and most phone and desktop music apps were never built to read its .snd header. Some apps also misread the bare .au extension as a different file type entirely. Converting to M4A solves both problems at once: AAC in an MPEG-4 container is the format iPhones, Android, and Apple Music treat as native, so the audio just plays.

What bitrate should I choose for the M4A?

For spoken-word or legacy μ-law clips, a low-to-medium setting (around 96–128 kbps) is plenty — the source audio has no detail above telephone quality to preserve, so a higher bitrate only enlarges the file. For linear-PCM AU with real music or wideband audio, 192–256 kbps gives a near-transparent result. If you're unsure, leave Quality Preset on "Very High (Recommended)" and let it match the encode to the source.

Does the M4A keep any metadata the AU had?

There's essentially none to lose. The AU container has no real tagging system — its header carries only an optional annotation string that most files leave blank. The M4A you get supports full MP4 tags, so after converting you can add title, artist, album, year, and embedded cover art in any tag editor (Mp3tag, Kid3, MusicBrainz Picard), giving these legacy recordings searchable metadata for the first time.

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

Files are uploaded over an encrypted connection, converted 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 30-second 16-bit/44.1 kHz stereo linear-PCM AU (about 5 MB) converted to a roughly 480 KB M4A at the default preset, while an 8 kHz μ-law AU stayed tiny and plays identically to the original on a phone. Need a lossless copy instead? Use AU to FLAC; for the most universal lossy format, try AU to MP3.