No matter how tough the phone or how clean you keep it, dust/lint and all manner of foreign objects make their way into your earpiece and corrupt your sound experience, especially in dusty/dirty industrial environments, and also just being carried around in your pocket where trapped contaminants survive the washing process and corrupt your earpiece.
With the varying degrees of quality of manufactures parts this is always going to be an issue, even with the so called flagship models.
The earpiece is essentially just a magnet and a magnet attracts dust, especially dust with iron or fine metal present. Mix this with a bit of lint and moisture and you can build Rome with the blockage it creates in your earpiece.
Here is a video with a few techniques to successfully clean out that blockage and restore your sound to what it should be.
This is aimed at IPHONES but works on all devices. (enjoy)
Credit to the video author for this tutorial.
Blue System Australia