Imagine if you paid for a book and there were appendices about characters or something extra, but written in a language you didn't know. It would kind of suck.
Edit: As a side note, these comments seem to be the internet equivalent of:
A: =~#$())##~
B: Sorry, what was that? I couldn't hear you.
A: Nevermind, it's not important.
Which I think most people would agree is incredibly rude and dismissive.
If you don't already know about it, you may want to check out the book Finnegan's Wake :)
Joking aside, I think you're right that there should be some kind of notice somewhere, in-game or out (even on the Steam page) that indicates that some parts of the game are difficult or inaccessible for players with a degree of hearing loss.
Even better might be to have an option in the game that turns on an additional visual cue that could augment whatever the original audio cue was. Actually designing it to make sense in the same way would of course be difficult, but I don't doubt that there are enough players that would appreciate that option.
(You have to play the game to get why he can't give hints)