Another 'plausible' explanation...would be changes in humidity and temperature and barometric pressure.
All these conditions affect the density of the air. Ask any pilot. Air is less dense when hot, humid, or high up
(like Denver). Aircraft performance deteriorates with less dense air. Why wouldn't it also affect the sound waves reaching our ears? Remember those train whistles on certain days sounding different?
My guess would be the more dense air would provide sonic benefits as the air molecules would be most closely associated with each other, and our speaker drivers are essentially 'bumping' the molecules....which then 'bump' each other...until reaching our tympanic membranes (eardrums) and bumping them to create small sympathetic vibrations which are then converted to chemically mediated 'electrical' signals carried by nerves to our brains. Wow!
On second thought, it probably is just the Audio Gods!