I am jailbroken on my iPhone 6s on 13.5. I have customized the UI sounds such as Siri sounds, the lock sound, the charger connected sound, and a few more. What erks me is that Apple doesn't let you customize the VO sounds natively with nonjailbroken iOS devices. They haven't changed those sounds since iPhoneOS 3, when VO was originally released, and again in iOS 11.1 with the volume sounds from the Mac. I would love to see new VO sounds in iOS 14, or a sound schemes manager like the one on Windows where you can customize the sounds of any aspect of the operating system you desire. However, I will send an email to Apple Accessibility Support and post their reply.
TL;DR: Seriously we need new VO sounds. ]