Does "ん" really have a dakuten version?
So my reasearch has been a little bit unusual for this one, but I found the following from reddit's r/learnjapanese:
Dakuten/handakuten on kana where they would normally not be found can imply a muffled, nasal, or slurred sound.
I also found some sources on this stack exchange as well. Their question was about dakuten being applied to あ.
As @naruto notes:
あ゛ used to be commonly used in manga to express an exclamation — "aagh!" or something like that. I feel it has become less common (if not rare) these days. え゛、い゛、お゛、の゛ and so on are sometimes used in a similar way. You may see them in casual blogs and tweets, but never in formal documents.
On Wiktionary, I found that ん゙ can signify a guttural "hmmm?"
Finaly, on Wikipedia, I found the following:
In informal writing, dakuten is occasionally used on vowels to indicate a shocked or strangled articulation; for example, on あ゙ or ゔ. Dakuten can also be occasionally used with ん (ん゙) to indicate a guttural hum, growl, or similar sound.
I believe that you're looking at something that is informal. For the most part, it looks like someone is saying "hmmm?" in a guttural, excited, or muffled matter, depending on the context.