Hi there! Sorry for the late answer!
If you are trying to use with MZ, make sure you also have my core plugin somewhere above the Sound Test plugin on the plugin manager.
https://hakuenstudio.itch.io/eli-book-rpg-maker-mv-mz
The same rule applies for the MV, but the core plugin and the most updated version of the Sound Test is here:
https://hakuenstudio.itch.io/eli-book-rpg-maker-mv-mz
For both MV and MZ, you can try the sample project to see the plugin working there and compare with your configuration.
https://hakuenstudio.itch.io/eli-sound-test-for-rpg-maker
If the problem persists, try testing the game again, but when you get to the map scene, press F8 or F12. A browser window will open. Then, go to the CONSOLE tab. Send me a screenshot of what is in there.