I feel it actually improves gameplay, if any. Loosely speaking, you see the monster, you haven't opened a door since the last time you fired a shot, the monster reacts; direct visual feedback. As the player has more information available, the variety of ways in which you can challenge his skills increase. As he has a better view of the big picture, you also have more control over the exact level of difficulty you want to achieve.
From the perspective of a player, is it really obvious when all the monsters are deaf, and does it dampen the gameplay for you when you are aware of it?
Comparatively, soundblocking can be great for immersion, but most gameplay situations it creates are likely to rely on surprise and obfuscation. You don't know where the monsters come from exactly, and likely, you don't know where the soundblocking lines are. There's some level of guesswork, and there will always be some level of guesswork unless you crack the map open in an editor.
It's not necessarily bad, but I think it tends to lead to more conservative play and homogeneous situations. As the player you want to save resources for nasty surprises, as the mapper you have to hold back on the amount of pressure you can apply on the doomguy.
Of course this isn't exclusive to soundblocking, you could say similar situations can arise from instant floors, hidden monster teleporters, and so on, and I don't mean to imply using soundblocking lines at all will always result in the above. It's just, looking at the gameplay angle, just like you I've been racking my brain to think of anything truly interesting it could do, and so far I'm coming up empty.