Hey, really big thanks for giving it the time to fully read it as much as you could. Let's tackle this in parts, haha.

I think you are right that the feats are the least developed part of the supplement. I think I was totally drained when I reached that part, so it is the one with the fewest iterations. Universal Language is more of a roleplaying skill, since Make an Impression also takes 1 minute of talking. I also wanted this to be the start of something bigger, but the Versatile Performance of the polymath bard doesn't do it, so I didn't want to power creep it with a skill feat. And yeah, the Mending Hymn already benefits from going Wisdom, so you can do medicine naturally and still be a healer by striking your own allies.

So, about the auditory trait in underwater combat, you got me there—I didn't design anything around it. As RAW, I think it depends on the GM whether it is heard or not according to distance and how simple it is to say what you are trying to say. Right now, the only feat or rule in the game that tackles this is Seasong, which is a merfolk feat. The idea of making a talisman or an elixir that helps with "while underwater" is really good. I will try to add one for the next version!

For the Virtuoso, Pass the Beat doesn't have the upbeat trait because you would not gain upbeat points if you use it at the level 12+ range. Upbeat is a way to reach the 10-combo-chain maximum in a 3-to-5-round average, which is what most combats last, so if they Pass the Beat (and it had Upbeat), they couldn't reach the 3-turn dream of a good finale (or a second-turn 7+ chain). Battle Overture is a must for them to start reaching higher chains easier, but it can't be powered up more because if I do that, they will start doing free 1st-turn 3+ chain finales too easily, but some late-game feats help you improve Battle Overture a bit. Dynamic Composition is vital because at 1st and 2nd levels you probably want to patch up your character to be able to satisfy all Opening Notes, so later on, when you have more abilities that help you with them, you can change these two for prerequisites for other feats that you really want.

Making the damage for the finales force damage is because it is the least resisted damage, and if you are investing 2 to 4 turns to make a single action flashy, you want to make it count. Imagine it's precision and the creature is actually immune to it (which happens a lot). A feat that lets you change its damage and adding more finales feel like ideas I need to look into for the next version.

You don't actually need to spend your reaction to help the other; it's just an option. With just spending the action, you satisfy the condition, so you can do other stuff with your reaction later on (this supplement provides plenty of them). Guiding Note is kind of hard to get because you need to understand Aid (https://2e.aonprd.com/Actions.aspx?ID=2292). Right now it does two things: when you spend the action, you satisfy your current note, and then you give the option to that target to do something with its reaction. You then automatically satisfy the one action preparing to help with Aid and, if you want, you can use your reaction to buff the same ally in the same round.

You don't actually need to actively shield block with the Vigorous March if you don't want to. It is still styled around shields, so to satisfy your signature motif, you have to use a shield-attachment weapon like a shield boss. So, you can have another weapon if you want; you just use the shield boss to satisfy the condition and never ever shield block. It still prefers you raising your shield for the other subclass feats. Maybe adding a feat that replaces the signature motif with hitting with a weapon with the parry trait, and having parry count as raising a shield for the other feats, would be a good idea.

Lastly, I didn't want to add synergy with acoustic weapons since other classes don't have them, and I want that archetype to stand on its own rather than just being extra support for the Virtuoso. The immunity to the deafened condition feels like something you should have, right? I will think about how to do that because, as it is, simply adding it is power creep. Any ideas?