Weirdly, as an en_GB native, neither of these seems wrong to me. I suspect one is British and one is American, and too much TV has driven me to accept the American one.
replyBoth of these are common in the US. It doesn't seem like there's any strong regional connection.
replyJudging from a search, "play the piano" is about 5 times as common as "play piano", but there are also many sources which uses both versions in close proximity, apparently interchangeably.
"i play piano" - i know how to play piano
"i play the piano" - i am playing the piano
replythat is just my intuition as native english speaker, but the former cojld also mean the latter and i have seen it used like such before.
With context these can mean different things. You might also play a piano, or hire either a player piano, or a piano player.
replyIf you are answering which instrument you play, it's the former, but if we are talking the profession it's the latter, because you don't say then that you play a piano.