In the best case, good music on hold should be voiced by a professional speaker. This will ensure that the speech is intelligible and reaches your callers with the right tonality.
The right sound format of the music also plays a role. Most telephone systems require one of the following formats:
CCITT a-law
CCITT u-law
MP3, 128kbps, mono
WAV, 16bit, 8kHz, mono
You can easily convert the formats yourself using an audio program, such as Audacity. If you need help with format conversion after purchasing on audiocrowd, we are of course available.