The top microphone meshes and bottom speaker grilles get clogged over time with earwax and fine debris, making the sound quieter and causing people on calls to complain that they can barely hear you.
The main rule — no liquid inside and no sharp objects: needles and toothpicks push through the membrane, and alcohol breaks down the adhesive layer. Use a dry soft brush with long bristles and carefully sweep the dirt from the centre outwards. For stubborn buildup, a special sticky cleaning putty for earphones works well: it gently pulls grime out of the tiny holes without leaving any residue.
If the sound has already dropped noticeably or one earbud is quieter than the other, the dirt has likely worked its way past the mesh, and without careful disassembly the problem won't be solved. Inside, everything is held together by miniature ribbon cables, so it's better not to try pushing the dirt deeper, but to leave the cleaning to those who do it with jeweller-like calm.