"I don't see how they can with most of my pieces, but I think it's unfortunate that they can through familiarity with flashy performances of a great deal of other music"

This quote by Peter Maxwell Davies is describing the problem of audiences to appreciate his pieces of music. He thinks that it is regrettable that people are more acquainted with fancy performances of other music, as this makes it harder for them to appreciate his own music. He is suggesting that individuals are most likely to be drawn to music that is more flashy and entertaining, rather than music that is more complicated and needs more effort to comprehend. He believes that this familiarity with flashy performances of other music is avoiding people from valuing his own music. He is revealing his disappointment at the reality that individuals are not able to value his music as much as he would like them to.