It is illegal to play copyrighted music for commercial use (which includes your on hold programming) without paying royalties. Music provided by Holdcom use is royalty free because Holdcom licenses it for Message on Hold usage.
Website: https://www.onholdmessagingdirect.com/blog/phone-greeting-and-on-hold-message-script-templates/

For those still using analogue or digital phone systems, voice on ISDN circuits has a bandwidth of around 4000Hz. The audio from an analogue phone is sampled at 8000Hz, and because of the nyquist theorem, must be low-pass filtered to 4000Hz. So any frequencies in the audio beyond 4000Hz are lost. Interestingly, the human ear and mind are good at recognising human speech, so this is considered "toll-quality" voice, i.e. good enough to pay for. This is the reason why certain vocalised letters are difficult or impossible to tell apart on the phone. For example, the differences between B, P, T, C, etc. are almost entirely above 4000Hz.
One of the most valuable benefits of a Message On Hold program is its ability to help your customer service representatives. As a starting point for your Message On Hold, consider adding frequently asked questions to your on hold script. Ask your customer service reps what questions they hear again, and again, and again, they put those answers on hold. This way, you might answer questions before even talking to a customer.