You can customize the hold music.
Method 1) In your in-queue flow, look under Settings -> In-Queue. You can select a different prompt (including user prompts).
https://help.mypurecloud.com/articles/hold-music-action/
Method 2) The hold music prompt is a system prompt, so you can upload a new .wav file for it. This will affect all in-queue flows (that you have not used method 1 on ).
https://help.mypurecloud.com/articles/system-prompts/
One note: this will not affect non-ACD queue hold music, such as when an agent puts a caller on hold--the caller will hear the default music that apparently sucks. We do plan on fixing this, but it's not on our 1H17 roadmap.