The first one. Basically 'cause while you do play one single violin, the qualities of playing it isn't exclusive to that one, you can play the same with a different violin.
Hopefully that's not too confusing :)
Like what Might Mig said, the first one is "I play the violin" which makes more sense, rather than "I play a violin". Because you play it in general, not really a specific one.
Oooh thank you! I get it now!