[GEEKY] iPhone earphones revisited

The earphone/microphone that came with my iPhone is acting odd. First I thought it was a problem with my ear, then with my iPhone, but it seems to be a problem with the earphones, which I never thought of as something that could actually fail, except if I broke it physically.

The sound quality is just fine, but it only gets 1/2 as loud as it should. How do I know? If I plug in a pair of brand-x earphones, I get the right sound level. Is this a plausible failure mode for the earphones?

P.s. Have you noticed that all these earphone pairs, the apple ones, but other brands too, have an unexplained tiny little clip along the wire in which you can slide another part of the wire? Nowhere is the purpose of this thing explained, but yet they all have it. What is it for???