This (and more) information can be found by clicking the help button in the web GUI.
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Audio File Format
The audio file must be in the following format:
WAV, A-law or m-law (G.711), 8 kHz, 8-bit, mono.
Tip: When extracting a file from a CD (for a example, from your corporate Music on Hold CD) using a "CD ripper" application, choose a sampling rate as close as possible to 8 kHz. This should help prevent audio distortion introduced when converting a WAV file from a high sampling rate to a low sampling rate.
When an audio file is downloaded, its audio amplitude is normalized by the controller to ensure uniform sound level and compliance with telecommunications regulations.