Yes it has been fixed
Another somewhat recent thread about this kind of thing: https://forums.macrumors.com/thread...ed-thinking-of-getting-an-iphone-7-7.2030810/Has the hissing issue been fixed or is it still a hit or a miss? i am about to pull the trigger on an iPhone 7 or a SE and was just wondering if the older issues have been sorted out.
i can't speak for other's. But i never encountered this problem on both of my iPhone's (iPhone 7 :- 6 months old and iPhone 7 Plus bought few days ago)