Howdy!
I wanted to provide some insight into this issue. Specifically, while in instanced content (dungeons, raids, scenarios, etc) the sound effects from other players should be significantly reduced and quieter than your own. It could definitely be that you can not hear the sound effects from other players at all, depending on what is going on in the instance and general sound settings.
The reason for this change was so that your own spells / abilities do not get drowned out by all of the other player’s casts. It was an intentional change, and there is no toggle option to revert it to how it was previously.
If you have feedback on this change, we highly recommend providing this to our developers Here.
Thank you.