Question: how to use pxc 550?
Answer: Just tap once on the outside of the earcup to play or pause your music. ... 
Tap and hold the touchpad for 3-4 seconds to get a PXC-550 battery charge/status percentage softly whispered in your ear. ... 
Sennheiser's free downloadable app called 'CapTune' makes these headphones even more feature-rich.