Sound Communication: The Holdcom Blog

File Format Definitions for Your Audio Program

On occasion, Holdcom will encounter an unusual file format from client-supplied music. With help from Holdcom's Audio Produers, today’s blog is going to present several audio file formats we see in Message On Hold programs and voice prompts and what to do when encountering an unknown format.



Waveform Audio Format is one of the most common audio extensions. It stores data in chunks, either compressed or uncompressed using linear pulse code modification (standard audio file format for CDs). Wav is the leading audio format in the audio industry today.


Moving Pictures Expert Group (MPEG) Audio Layer 3 is another common audio extension. These files erase frequencies that are inaudible to the human ear, presenting a more compressed wavelength. Producers that are mass marketing songs usually increase the high and low ranges and the overall volume, ignoring the mid-range that can be “rounded out” by the brain upon interpreting the audio.

An example of the differences between both formats. The top is .mp3 and the bottom is .wav; you can tell that .mp3 is a skeletal version of the full bodied .wav:

audio extension, wavelength, voice over

.Aiff (.aif, .aifc)

Audio Interchange File Format: Apple codec for audio. It is uncompressed and lossless (exact original recorded data to be extracted from compressed data, without distortion or cutting), like Lossy MP3. However, it takes up an inordinate amount of space (10 MB for 1 minute of recorded audio at 44.1 Hz and 16 bit depth).

.Vox (Dialogic ADPCM)

File format that works best with digital voice at a low sampling rate – what Holdcom uses for telephone systems, message on hold, business voice greetings, virtual tours, IVR – the whole gamut.


Digital systems interpret audio through taking “snapshots” of the total wavelength – the higher the sampling rate; the more of the wavelength is present in realizing the audio, thereby increasing the quality of the audio. However, this takes up more space. The low sampling rate of .Vox, but still providing good quality, makes it perfect for conserving space on telephone systems.

But what to do when you see a .cda file, or another unknown extension? The safest bet is to convert the files to common formats. Two well-known programs are Sound Forge and Audacity.

A .cda file contains no audio information – it is merely a “location” tracker for where the streaming data is located on an audio CD. Therefore, conversion is required.

Of course, the file format you use is dependent on the audio equipment used to play it, so checking your Message On Hold equipment or phone system's specifications should always be your first step.

Tags: resources, equipment, file formats