We are experiencing the same issue. We’ve tried everything. Nothing makes the EQ work. Not even when putting the sound mix class as passive in the sound class. Volume works fine. But EQ refuses to work. I’ve been looking all around the web for answers and I seem to have found a pattern of people with the same issue and not one single solution. So my guess is that’s it’s just broken and bugged. This sucks bigtime because I really wanted to muffle the sounds when you go underwater in our game. I wish there was a solution for this. 
I remember doing this in an old version 4.18 or something where this worked perfectly. Please someone from the Unreal team have a look at this. This is a big feature that is currently not useable.
EDIT: I have found a solution!
You need to enable this setting on your sound class if you want to use the EQ of a classic (deprecated) Sound Class Mix. It’s recommended to use Sub Mixes instead now as they have an EQ as well.
