Pretty much what the title says. In the main series, sound-based moves (Hyper Voice, Boomburst, Bug Buzz, etc) are able to hit enemies that are protected by a substitute, but this is not the case in PMU. I don’t think making this change would make sound-based moves too overpowered or anything, as Substitute isn’t common in dungeons in the first place, but it would give them a little niche that might make them a little bit more appealing for 'mons that get other ranged options, given that as of now, unless you’re a member of the Popplio line, the only “special trait” of sound-based moves is that they’re blocked by Soundproof. I dunno, maybe I’m the only one who feels this way, but it’s just a change I’d like to see.
I believe Kirk tried to implement this a while back but ran into issues, so the current behavior is certainly not due to a lack of desire to make the change.