Event reminder sounds don’t always work

For some reason, my event reminder sounds don't always work. The reminder popup window always works, but only sometimes comes with a sound, so I don't always notice it and have been late to a few meetings as a result. In my outlook preferences, under sounds, the reminder sound seems to work just fine, just not when I actually need to be reminded of something.

Two questions: Any possible causes for this? Is it possible that sounds only play under certain circumstances, such as when snoozing or not snoozing, or some other combination of actions?

Thanks