Free Stock Music provides free music for download and even has a tag for music on hold. Beatsuite is a great resource for royalty-free hold music, and they also offer professional voiceover services for on-hold messages. Easy on Hold is another great site, and they even break down how their system works with all types of phones, including VoIP!
Website: https://info.umkc.edu/hr/wp-content/uploads/2017/04/Telephone-Etiquette.pdf

If you're not actively planning content for your on hold program, and, say, play the radio on hold, you're not controlling what you're playing on hold. You're at the whim of a Radio DJ or program manager, who might play music that's unappealing to your callers or who might be producing and playing ads for your competition.
In order to get started with custom on hold messages, you will need to determine your phones MOH (Messages On Hold) capability. If you’re using a traditional pbx business phone system, you’ll need an on hold media unit to play the audio. If you’re using an ipbx or hosted voip set-up then all you need to do is upload the audio file as no media device is required. And, if your a small/home office business using a traditional off-the-shelf 2-4 line telephone then you’ll need the media device and an on hold module adaptor in order to make your standard analog phones on hold message compatible.