Free online AU to MP3 converter

Older Unix applications, voicemail systems, and archived sound assets may store audio as AU, while most phones, media players, editors, and sharing services accept MP3. Conversion creates a smaller, widely playable copy without changing the original recording.

What AU files contain

AU, also called Sun/NeXT audio, is a legacy audio file format associated with Sun Microsystems, Unix workstations, and some Java-era software. AU files usually use .au; .snd is another common extension.

Many AU files contain 8-bit μ-law telephone audio, but the format can also store A-law, signed PCM, and other audio encodings at rates such as 8 kHz, 11.025 kHz, or 44.1 kHz. Modern consumer players may not recognize AU even when the audio itself is standard PCM.

Why MP3 is used

MP3 is a lossy compressed audio format supported by most media players, phones, car stereos, browsers, and upload services. It usually produces far smaller files than uncompressed AU audio and is appropriate for listening copies and distribution.

For mono speech, 64–96 kbps is normally sufficient. Use 128–192 kbps for music or higher-fidelity source material. Encoding at 256–320 kbps can increase file size but cannot restore detail missing from a low-rate or telephone-quality AU recording.

Convert the file on 101convert.com

Upload the .au or .snd file to 101convert.com, select MP3 as the output format, start the conversion, and download the resulting .mp3 file. The service can be used free without installing software.

Keep mono output for an originally mono voice recording. Converting mono audio to stereo only duplicates channels and increases the MP3 size. Do not rename an AU file to .mp3 instead of converting it; changing the extension does not change the encoded audio.

Convert with desktop software

In Audacity, open the AU file with File → Import → Audio or drag it into the project. Confirm that playback is correct, then choose File → Export Audio, select MP3, set the filename, and choose the required bitrate.

FFmpeg can convert common AU variants, including μ-law, A-law, and PCM:

ffmpeg -i input.au -c:a libmp3lame -b:a 192k output.mp3

For a mono speech recording, retain one channel and use a lower bitrate:

ffmpeg -i input.au -c:a libmp3lame -ac 1 -b:a 64k output.mp3

Quality and compatibility limits

An 8 kHz μ-law AU recording has telephone-bandwidth sound. Higher MP3 bitrates or resampling it to 44.1 kHz will not improve clarity; preserve its native sample rate unless a receiving device explicitly requires another rate.

MP3 supports low sample rates, but some older hardware has limited support for them. If a destination device rejects an 8 kHz or 11.025 kHz MP3, create a separate resampled copy for that device rather than replacing the original conversion.

Keep the original AU file when it is the best available source. MP3 encoding discards data, and re-encoding an MP3 adds further loss. If a converter cannot open the file, inspect it with ffprobe input.au; uncommon AU codec variants may require a current FFmpeg build.