The ear has three parts: Outer, Middle, and Inner
A blockage in the outer or middle ear will result in less sound pressure transmission to the inner ear-- things sound muffled or have less volume. There's not enough evidence or data to support suggestions like auditory fatigue. Changes in blood pressure (working out) can and do directly impact the inner ear and all of its parts. Hence why sometimes we feel dizzy when standing up quickly after sitting for a long period of time.
More than likely the problem is not caused by earbuds. It takes years, sometimes decades, to actually block the outer ear or the ear canal with wax and debris. Typically an impacted ear will give a flat conductive hearing loss. A partially impacted ear will give a high frequency conductive hearing loss that doesn't affect the low frequencies because the low frequencies can easily bend around any small opening.
Solution: Clean your ears with a peroxide solution (Debrox is sold OTC at the drugstore). If that's not an option, equal parts vinegar, water, and rubbing alcohol will work too. That will soften any debris and if it doesn't come out on it's own, it's easier for people like me to remove.
A change in the middle-ear system is actually more common due to sinus inflammation in adults. A hearing loss will often present with the feeling of pressure or fullness-- sometimes even tinnitus (that's an entirely different explanation that would take days to explain). Typically called an 'ear infection', it is the generic result of eustachian tube dysfunction. If the eustachian tube isn't working a positive or negative pressure (Re:atmospheric pressure) will occur in the middle-ear. This changes the mechanics of the middle-ear drastically. Ultimately it changes the resonant frequency of the mass-stiffness system creating a conductive hearing loss.
Solution: Wait it out, ~85% of ear infections are resolved on their own. If it is painful, go to the Doctor. Pain=infection and a round of antibiotics are better than the infection spreading and causing many more problems (believe me, ear infections will really mess a person up financially, emotionally, and physically). In adults, I usually see sinus medication given before an actual antibiotic.
Hope that helps a bit.