Boxing bell - 8 dB quieter, and now it sits under the music. It rings at the start of every round, when a boss gets back up, on every unlock and on the win, and a player's review said it was too loud. It was. Everything else is 2 dB quieter too, evenly.
Loading - the game no longer does all its warm-up work in one frame. Everything that has to be prepared before the first punch is now spread out, a little per frame, so the hitch you could feel while the title was up is gone. Nothing about the fight changed; it just gets there more smoothly.
Startup crash on some machines - the game could fail to start, with nothing on screen and no message. Two systems were being created in whatever order the engine felt like, and one of them needed the other to already exist; when the order came out wrong, it took the whole startup down. It does not depend on order any more. If the game never opened for you, try it again.
OBS capture layer closes the game, even on the latest OBS - this is a known s&box + OBS issue, not a bug in the game, and it happens even if your OBS is up to date and closed. OBS installs a Vulkan capture layer on the system, and that layer loads into s&box whether you are recording or not; when it misbehaves, the game shuts down at startup. Updating OBS usually fixes it. If it doesn't, set the environment variable DISABLE_VULKAN_OBS_CAPTURE=1 and try again. If you want to check it yourself, the crash report s&box collects names graphics-hook64.dll, which is that layer's hook. It took down my own editor too, while OBS was updating in the background.