PacOManiac :
It might be the inside of the earbuds. Sometimes an earbud's meshing can be indented. Which means the meshing on the earbud is push inwards, touching the cone, which is bouncing back a majority of the sound produced; resulting in a lower sound. Use something with a very small point,(ie. Swiss Army Knife, Fine Tweezers) to try and pull the meshing out. You can tell if it's indented by using a bright light to examine the meshing. Hope this helps.

Hey thanks! I was having problem with my Sony earphone (just bought about a month). Turns out smth wrong inside my right earbud, I just took a pin and try poking slowly. And the sound is all good! Thanks!