Free online MP3 to VOX converter

A telephone system, voice modem, or legacy speech application may require audio in VOX instead of MP3. Converting the file creates a small, compatible voice file for that specific system, but the destination’s required VOX parameters must be known first.

What the MP3 format is

MP3 is a lossy, compressed audio format used for music, podcasts, voice recordings, downloads, portable playback, and files created by phones, media players, screen recorders, and audio editors. An MP3 stores encoded audio with a sampling rate, channel count, and bitrate; conversion to VOX may resample it, reduce channels, and apply another lossy encoding step.

What the VOX format is

VOX usually refers to a headerless Dialogic/Oki ADPCM audio file, commonly configured as 8 kHz, mono, 4-bit ADPCM. Older telephone systems, IVR applications, voice modems, and embedded speech devices use this format because ADPCM produces small files that require little processing power to decode.

A standard VOX file normally has no header containing its sample rate, channel count, encoding, or duration. The importing software must already know those values. The .vox extension is also used by some applications for other raw audio encodings, including IMA ADPCM and PCM, so confirm the required specification before converting.

How to convert the file

Using 101convert.com

1. Open the MP3-to-VOX converter on 101convert.com.
2. Upload the .mp3 file.
3. Choose VOX as the output format and start the conversion.
4. Download the resulting .vox file and import or test it in the target telephone or voice application.

This option is suitable when the destination accepts the converter’s standard Dialogic VOX profile. If the destination specifies a different sample rate, channel count, ADPCM variant, or nibble order, use a local converter with explicit settings instead. The file can be converted on 101convert.com for free without installing software.

Using SoX locally

SoX can decode MP3 when installed with MP3 support and write Dialogic/Oki ADPCM VOX. For the common 8 kHz mono specification, run:

sox input.mp3 -t vox -r 8000 -c 1 -e oki-adpcm output.vox

Replace input.mp3 and output.vox with the actual filenames. Replace 8000 with the destination’s required sample rate if it specifies another value. The -e oki-adpcm option selects the Dialogic/Oki ADPCM encoding rather than assuming that every file using the .vox extension has the same format.

SoX does not add a descriptive header to a standard VOX file. Check the output in the receiving application or device, because a file may contain valid audio but sound distorted or play at the wrong speed when imported with incorrect parameters.

Quality and compatibility limits

Conversion cannot restore detail removed by MP3 compression. Converting to the usual 8 kHz mono VOX profile removes high frequencies and stereo information, which is generally appropriate for speech but unsuitable for music or other high-fidelity audio.

Confirm the complete specification before delivery: Dialogic/Oki ADPCM versus IMA ADPCM or PCM, sample rate, mono or stereo, signedness, and nibble or byte order. Headerless audio cannot identify these settings itself. Keep the original MP3 or, preferably, a high-quality WAV master for archiving; use VOX as the delivery format required by a particular legacy system.