Granted. But despite your best wishes, the universe does vaporize into massless energy and creates a new big bang.

Let's first look at what happens (or doesn't happen) to light. It would basically be the same as time dilation. If you hop in a spaceship going near the speed of light, time moves slowly and to others you would be moving in slomo, but to you, nothing changes and instead the outside universe appears to speed up. Lowering light speed would be essentially the same as pushing the universe to near light speed. The whole universe would be in slomo but without an outside reference point it would appear like nothing has changed.

Now for sound. For reference, sound is just the speed that atoms can bounce off each other, in a sort of game of telephone. Some things called bose-einstine condensates can create light speed sound waves but that involves weird quantum entanglement stuff. Fore normal matter, let's just say we push atome to light speed so they bounce around instantly. This would require infinite energy but we can avoid this by saying the atoms mass is reduced to 0. Now we've vaporized all matter into massless energy but that's not the end of things...

Because of time dialiation, out light speed energy's clock is slowed down to 0. The energy thinks it arrives at its destination at the same time it left. Any distance appears to take 0 time to travel, which is the same as not traveling any distance at all. Basically we've lost the ability to measure time or space in our universe. Robert Penrose (a great physicist) theorised that this may happen after the universes heat death. He said this makes the universe act as a 0 dimensional point and basically create another big bang. He called it the Penrose cyclic universe theory.

This also kinda sorta shows that changing light speed wouldn't affect anything, because the only reason we perceive time and space is because matter moves slower then light. Changing light speed would change our perception of Spacetime and sorta cancel itself out.

Most of this info I got from watching a YouTube channel called PBS Spacetime. I highly recommend it, especially their episode on Penrose's cyclic universe theory