Sentence:  Instead of her old confidence in the simplest , purest , most moving musical expression , Miss Schwarzkopf is letting herself be tempted by the classic sin of artistic pride - that subtle vanity that sometimes misleads a great artist into thinking that he or she can somehow <WSD>better</WSD> the music by bringing to it something extra , some personal dramatic touch imposed from the outside . 
Meaning of the word marked with <WSD> tags:  to make better