If appreciating music is entirely physical, then why would I like a certain piece in a certain setting but not in another? Or why would I like it on the first playing, present replay, not like it on the second?
It's no different except in degree from other physical measurements. In the lab we take a series of measurements of supposedly the same phenomenon and then subject the data and ourselves to the most tedious least-squares processing to get some idea of the range of values associated with the phenomenon. That is for a phenomenon exhibited by a system of a low level of complexity. A system of a higher level of complexity could show relatively similar values most of the time, but then throw off a wildly different value from time to time. When the level of complexity reaches the level that the system needs to treated as organic rather than statistical, then the values will vary greatly. The functions of the mind are a phenomenon of a system of the highest level of complexity that we know, so the varying results, liking a song one time and not the next are completely natural. Sometimes you want a cheese omelet, next time a western omelet, it's how the physics of the complex works, organic physics. Ever hear of organic physics? It's like biophysics, an extension of ordinary physics, but including phenomena due to the most highly complex organizations. You might even include politics, sociology, and psychology as subsets of organic physics. It's easier and gives you a coherent scientific tool top to bottom. Kind of like philosophy, but scientific.