You can customize sounds in Windows Mobile and Windows PC.
On Windows mobile, put the .wav sound files into “My Documents” on the device and choose Options > Control > Change... buttons to select your sounds.
On Windows PC, put the .wav sound files in "\program files\Zello@Work\Snd." Then customize alerts.
At this time, it's not possible to change the sounds from the ZelloWork web console.
Tip: The audio files must be .wav.