<font color='#000000'>Dude, that's just life!
I mean, don't you wake up some mornings and think "it's great to be alive" and others it's "WTF, another #@$! day"?
Seriously, state of mind, level of fatigue, and maybe even what you ate/drank or how well you slept all affect your senses. It sounds different from day to day because you're hearing (more precisely, perceiving) differently from day to day. It can even change with the time of day. Pharmaceuticals, whether prescription, over the counter, or "recreational" can obviously have effects on both hearing (narrowly defined as what's actually happening within the structures and nerves of your ears) and perception, too, of course.
Or...it could be good and evil spirits! Myself, I have a little altar made of old CDs and LPs, candles, and incense sticks with pictures of Beethoven, Mahler, Thelonius Monk and John Lennon next to my system. Every few days I sacrifice a chicken and pour a shot of vodka over the altar...</font>