I just noticed the event sounds doesn't work when you disable the domain reload in "Project settings -> Editor -> Enter play mode settings". It works the first time you enter play mode but not after that unless you restart unity.
I tested this on unity 2020.1.10f1 and Master audio 4.0.
Well it just doesn't work with every event i tried.
Here's an example project: we.tl/t-O3ATIwnMMB To not leak the master audio source code you'll have to import it by yourself ,restart unity and load the sample scene for the project to work (i used unity 2020.1.10f1 and master audio 4.2.9)
When domain reload is enabled everything is fine but when it is disabled nothing works anymore
I just realized you're over a year behind on updates. I always ask that people with bugs to report first upgrade to the latest version of Master Audio to check if it's fixed before I take a look, since it is often the case.