We'll deal separately with two types: 1) recorded voice messages and 2) music files
Recorded voice messages
Good quality phone experiences usually depend on the quality of voice recordings. You can book a "voice talent" or "voice artist" with experience of IVR and call-flow message recording from numerous sites.
Some specialist sites for voice artists:
You can also find voice artists on freelancer sites like:
Music audio files
Music frequently needs to be licensed. If you wish to get royalty-free music, there are various sites to search for such content. Note that royalty-free does not mean "free", it usually means that you pay a onetime small fee and no regular fees afterwards. Please check that you actually have the right permission or license to use the music.
Also, it is possible to cheaply license music from some sites.
Examples of music sources for on hold, waiting and other call-flow uses:
Note: We provide these as examples for you as a reference. There are many other similar sources. Please let us know if you use any that you are particularly happy with, or ones that you are not, so that we can update the list.