OP, drop it - the guilt. It's a very small detail and the moment will pass. However, as DIL is not your place to comment again. Make peace that this family (DH's family) will deal with it, ignore it, or whatever.

Can you talk with him about wanting him to be able to hear your daughter instead of telling him what to do about it? As other people have pointed out, hearing aids may not be the solution.

IMO that was a rude way of bringing it up, I don't doubt he needs hearing aids but it's not your place.

I think you were right for speaking up. I think when things like this effect other people you should speak up. We have lots of trouble with my grandma and dad's hearing. I think if no one had spoken up they never would have gotten hearing aids.