Continually there are changes in our environment - everything from changes in temperature, changes in humidity and air pressure to fluctuations in microwave transmissions - that all have a slight affect on sound and an additional effect on how well we hear it. All of these same things, along with allergy flair-ups, inflammation, blood pressure fluctuations and even minor infections, can have an affect on inner ear and sinus pressure and how we hear at any particular time.
There are some nights I wake up and hear the tic-tock, ticking of both wall clocks in the rooms adjoining my bedroom - almost to the point of wanting to cover my head. Other nights I can barley hear them and sometimes, no mater how hard I listen, not hear them at all.
It could be an unaccounted for change somewhere in the chain of your audio system, but, just as likely, some type of change in your environment or in your self....Jim