I think she also contradicts herself a bit here, and it's funny because she's so sure her opinions are correct, but yet she contradicts herself. Colonel Brandon is the only one who has any taste because he's the only one who listens to her, but on the other hand, maybe he's too old to have any feelings about anything, so he's just sitting there quietly because what else would someone that age do? And Colonel Brandon's attentiveness shows that he has some taste, and yet he is not in raptures over her performance as one ought to be if one had any taste.

"His pleasure in music, though it amounted not to that extatic delight which alone could sympathize with her own, was estimable ..." Here is Marianne thinking in absolute terms again--only ecstatic delight is the right way to show pleasure in mustic. And yet it was the others who applauded so much and loudly admired her performance, but she finds them horribly insensible compared to the Colonel.

The part about hearing her without being in raptures, reminds me of what Marianne said about Edward admiring Elinor's drawing in Ch. 3 "It is evident, in spite of his frequent attention to her while she draws, that in fact he knows nothing of the matter. He admires as a lover, not as a connoisseur. To satisfy me, those characters must be united. "