Well, this is me again…
I was wondering, previous days, that when I mention “I play the guitar” (talking 'bout my self) I said so. My concern is: is there any explanation when saying “I play the guitar”? As you’ve noticed one could say specific things like “the earth”, the sea" or “the sky” because they are specific ones and unique. And then I wondered “why the guitar”? …May I say " I play guitar"? Well, just let me know what you know… thanks…
See ya!
Hi Sergio
Good question. I think this may a a bit of a “gray area”.
If you just want to talk about your general ability to play a particular musical instrument, then I think you could say it with or without “the”.
But I personally would probably use ‘the’: I play the guitar.
Amy
I agree. I, too, would say, “I play the guitar.” It just sounds better to me. Saying “I play guitar” seems a bit awkward.
Hi Sergio
Sometimes you just have to accept the fact that there is no “logical reason” for certain things in a language. 
Sometimes the best explanation is: “That’s just the way it is.”
I would also say “play the piano”, “play the flute”, “play the harp”, etc. So, it isn’t just a question of playing “the guitar”. And since that’s how I would say it, that’s why it sounds better to me. 
But, as I’ve already mentioned, this seems to be a “gray area”. You will definitely hear it both ways — with and without “the”.
There are also some “the” differences between British English and American English. For example, in American English I would always say “I had to go to the hospital.” (I believe in British English you would omit ‘the’.) Why do we Americans say it that way? I have no idea. :shock:
Amy