Fixing something that was already in the game, but was broken for some reason, is generally okay. For transparency’s, you could mention it on the game page.

I would consider that a bug fix. Which IS covered by the change agreement. So yes. Go right ahead. (If you can, just edit your original post.) I will be looking forward to seeing your game when you upload it again.

People usually do this, but not as a replacement for their competition submission. Instead, they reupload the file under a different header, usually something like “Post-Compo” in which they make changes. You leave what you finished during competition time up and that is what people judge. This new version is just an alternative like, “If you like my submission and rated it well, I’ve updated it and it’s even better now if you want to try”.

Oh, I didn’t realise it was fixing sound as opposed to adding it in. If it’s just broken, I’d say it’s probably okay to fix. I would mention it though, in the description, just in case.