Currently I am getting no sounds from this mod; I've used it before so I know what it adds and I'm not hearing it.

I have a bunch of mods installed but I'm not crashing so I don't know what's causing this to break.

This mod is incompatible with Resounding. And possibly some others, Resounding just makes it extremely quiet but at least it plays, there is another that's causing the sounds to not play entirely.

Regarding the sounds being muffled - that is a Resounding issue. ExtraSounds plays sounds the exact same way Minecraft's UI does, although I don't necessarily like it, as all sounds are positioned - most likely this is why Resounding trips up. For now I think this is an issue that needs to be resolved on their side.
Regarding sounds not playing entirely - I cannot reproduce this. A demo screen recording + logs would be appreciated if you're still facing that issue.

Never mind, I can reproduce this only with Resounding installed. Seems like an issue on their part currently. If you are able to reproduce this with only ExtraSounds installed, I'll reopen this.