So hiccup and all the other Vikings on Berk thought thunder drums were deaf but in riders of Berk we are family part 1, fish legs uses a thunderdrum call and one comes running. If they were deaf how would he hear the call.
That's just general inconsistency on RoB's part. According to canon, they are virtually deaf. According to SoD, they can hear vibrations like snakes, and may hear a little bit better underwater.
They can probably hear the natural thunderdrum calls because they're naturally loud, and fishleg's may have just sounded like a far-away call.
Since in RTTE, I think it ws just described that they didnt have good hearing... enough to block off a death song call, but can still hear to a certain extent
They are deaf because of their loud roaring. RTTE, and other sources list the Thunderdrum as "almost", or "nearly" deaf, so they can hear a little, but probably only sounds that are either deep enough, or loud enough for them to hear.
Edit: becoming deaf was kind of a defense mechanism against their roaring, so they would only hear things that are as loud, or louder than themselves.