I've also been having huge problems with this. My sounds.json file is in the correct place, and the command blocks are saying that they played the sound, but I just can't hear it. I've tested the file itself and it works fine. I'm just completely at a loss. I've also gone and looked at other people's maps that include custom sounds, and copied what they did exactly. The custom sounds in those maps work no problem, so it's not like I have sounds off.
I'm totally at my wits end ;A;