You can do it like a Chinese book publisher: "Dear author, your novel is so excellent that in comparison it would make our other authors' talents look small. We cannot take that responsibility, therefore to our regret we must refrain from publishing your work."

More occidental: "For our programming policy of a well-balanced mix, we need something different. Please do not take this as a judgment about the quality of your performance." (— "So what is it that you want? I can write it quickly!" — "We cannot possibly ask you to compromise your artistic integrity." Don't be specific about what you want, to avoid endless arguing.)

Instead of an audition, ask for a short recording, e.g. to be mailed as an MP3 file.