Free online GSM to AU converter

Source format: GSM

GSM is a compressed audio format originally associated with the GSM 06.10 speech codec. It is mainly used for voice recordings, telephony archives, voicemail systems, and older speech-based applications. GSM files are typically small and optimized for intelligible speech rather than high-fidelity music, so they are most common in environments where storage efficiency matters more than audio quality.

Target format: AU

AU is an older Sun/NeXT audio container format that can store uncompressed or lightly compressed audio. It is widely recognized on Unix-like systems and by many audio tools, especially for legacy compatibility. AU is useful when you need a simple, broadly supported audio file for technical workflows, archival exchange, or compatibility with older software that expects this format.

How to perform the conversion

The most reliable way to convert GSM to AU is with FFmpeg, which supports both formats and preserves audio data correctly during decoding and re-encoding.

1. Install FFmpeg if it is not already available on your system.
2. Open a terminal or command prompt in the folder containing your GSM file.
3. Run a command such as ffmpeg -i input.gsm output.au.
4. If your file has a different extension, use its actual filename, for example ffmpeg -i voicemail.gsm voicemail.au.
5. Check the resulting AU file in an audio player or with a media inspector to confirm it opens correctly.

If you prefer a desktop application, Audacity can also open many GSM files through supported import methods and export to AU when the required codec support is available. In Audacity, use File → Open or File → Import → Audio, then File → Export → Export Audio and choose AU as the output format.

You can also convert the file on 101convert.com for free, without installing any software.

Notes on quality and compatibility

GSM is speech-oriented, so converting it to AU does not improve audio quality; it only changes the container or encoding used for the output. If the AU file is saved as uncompressed audio, the file size may become much larger than the original GSM file.

Compatibility is generally good for legacy and Unix-oriented tools, but some modern players may have limited support for AU compared with more common formats such as WAV or MP3. Also, if the source GSM file is damaged or uses an uncommon variant, conversion may fail or produce audio artifacts. For best results, keep the original sample rate and channel layout unless you specifically need a different AU configuration.