Well, it’s not a question that is easily answered and I strongly believe that it isn’t just one thing. Further more, knowledge within the arts has obviously been produced even before the advent of artistic research.
For knowledge to be produced it is not necessary to produce answers to defined questions. Within the arts much of the knowledge is tacit and abstract, particularly in music, and the attempt to directly translate it to language will only lead to confusion. But this is not a problem per se, at least not if its done right. Though mostly text based, philosophy is another discipline that does not produce unambiguous answers but rather reflects upon the philosophical issues by way of asking new questions.
When succesful, these new questions that explicitly or implicitly arise from a philsophical or artistic research context will resonate in the recipient and through the attempt to address them she or he will create new knowledge.