Hi there!
My guess is that you are using the Eli Book version of MV or the Eli Book is below the Message sounds. So let's make sure of some things:
1 - You have the EliMZ_Book.js somewhere above the EliMZ_MessageSounds.js
2 - When the error happens, press F12 or F8, and a window will be opened, go to the CONSOLE tab and send me a screenshot of the error trace that appears there.
3 - Send me a screenshot of your plugin manager so I can take a look at the versions of the plugins.
If still the problem happens, build me a sample project replicating the error, upload it somewhere, and send me so I can download it and take a look.