I have a pair of CX-200..... i would see if you still have a warranty left? ( if bought from a authorized retailer i would contact Sennheiser).
I can't really comment on how to take it apart, ive not done that before esp. for something so small, repair would be quite difficult..... just thought i would point out another option even thou it doesn't answer your question about taking it apart. I cant see a way into my pair.
and of course check audio settings for your computer/ipod etc.... i'm sure you checked this but can't hurt to make sure your audio source is behaving correctly or the settings have not been changed.