When someone perceives a piece of art, two separate and equal entities interact. These entities are 1. the mind of the audience member and 2. the piece of art, which reflects the mind of the artist. The artist, having, in a sense, put part of himself in his work, is present in his art, speaking to his audience through his work just as truly as he might speak to a friend in person or through a letter. This is true whether the art is music, painting, literature, film, et cetera. Likewise, the audience member's mind actively participates in the work of the artist, simultaneously perceiving, comprehending, interpreting, and evaluating the art, taking his part in a conversation begun by the artist.
The artist and the audience are essentially equal partners in conversation, not interacting face to face but rather conversing by proxy as through a letter. The artist, of course, is like to the sender, and the audience to the reciever. The artist sets the terms of the interaction, as it were, by making the first move, laying the groundwork, conveying concepts and ideas to which the receiver must respond. As in written communication, furthermore, the audience member is able to - nay, expected to and must - respond. His response may, on the surface, only take form of thoughts which the art inspires in his mind. But what goes in also comes out: thoughts produce words and actions, which, no matter how small, form another type of response to the artist's work. Finally, the art may provoke a strong enough reaction that the audience member feels compelled to respond explicitly to it, whether through a comment to a friend or a piece of writing interpreting and critiquing the artist's work.
One piece of art may affect two audience members in different ways, just as a verbal or written statement may have differing effects upon different audiences. Perhaps the meaning is clear to both audience members and the difference comes in their emotional or intellectual reaction to the message, or perhaps the meaning is ambiguous and the difference comes in the interpretation. Perhaps the artist had one of these perceived meanings or reactions in mind as the desired effect of his art; perhaps both, perhaps neither. But one thing is clear: these effects were caused not merely by the piece of art and not merely by the mind of the audience, but by both. The artist's work is the constant, the audience's mind the variable.
Thus, it is unfair to say, "the meaning and significance of art is determined by the artist," or "it is determined by the audience." It is determined by both. The artist and the audience are equal partners in interpretation and the creation of significance. It is also unfair to say, "value in art is objective" or "it is subjective." It is both objective and subjective, the former element flowing out from the artist into his work and the latter a function of the audience that interprets it and experiences its various effects.
Art, on it's basic level, is an essential part of the human experience. It is communication in its vaguest but most poignant form. It is profound interaction between strangers. It is objective and subjective, like life, and like life is both concrete and abstract. Art is conversation, it is dialogue, discussion, debate in the language of the angels. Art is the way we humans find meaning, the key by which we access and understand our world.