I noticed this passage: 'She played a great deal better than either of the Miss Musgroves; but having no voice, no knowledge of the harp...' etc.

I think JA meant this metaphorically as well as literally - she is 'only Anne', the one who is never listened to, whose wishes are never attended to. She has no voice... in anything

A little further on in this paragraph, it says: '...excepting one short period of her life, she had never...known the happiness of being listened to, or encouraged by any just appreciation or real taste. In music she had been always used to feel alone in the world...'