Yes of course, when you take things to the microcosmic level, science can't hold onto fact. And of course, as you say, what we believe to be true is based on fallible instrumentation, etc, and semantics can prove our scientific facts speciously founded....yeah, all that is true enough, but you can argue that the chaos and error at the microcosmic level doesn't alter the macrocosmic mechanics of what we believe enough for us to consider there to be no 'facts'.
Disregarding that though, you still have your a priori knowledge. Isn't it a fact that 1 + 1 = 2? I would have said so.
As concerns opinions, I'm still with you in that you can't bring fact to bear on qualitative analysis of something like music. The idea is simply inane.