Could it be "practice"? Even for musical instruments (uczę się skrzypcach?)
Not really. In English you can learn an instrument, but in Polish you need to learn to play the instrument. Maybe it would be acceptable in colloquial speech to forget the playing part, but I wouldn't say that. Even then, it would be "uczę się gitary/skrzypiec" (Genitive). Better stay with "uczę się grać na skrzypcach".
And "practice" when you mean you're learning, is "ćwiczyć".