DM was appalled when I revealed I couldn't hear the differences between the different characters on Coronation Street. She had moved from Sheffield to Lancashire (various bits) as a child but I was brought up Dahn Sahf.
I live in the north now. I can hear Warrington/Manchester/Cheshire/Bolton now, let alone Lancashire v Yorkshire.
Re Sherlock's violin, I think there are two problems.
(1) for set reasons, it would be complicated to have him actually play the violin (would be picked up by the wrong microphones). The bit that looks wrong is the fact that the bow barely touches the string and he's holding it in a fist so you can tell he's miming, just like when singers' throats don't move so you can tell they're miming.
(2) Sherlock is meant to be crap at the violin but not notice. That's the whole point. The sound they dub on is more like what he hears than what John hears!
So ... I'm on the fence. I've previously established those thoughts for myself so I don't throw things across the room during violin scenes.
In general, faked music is excruciating. Ditto faked anything skilful that the viewer has more experience of than the actor - fencing, rowing, skiing, etc.