Is there something unique about the HTC Titan headphone port? Neither my iphone headphones or the $40 Skull Candy headphones with remote and mic that I bought support the volume feature on the HTC Titan, however work just fine on my old 3GS. The headphones that came with my wife's Focus do work on the Titan.
iPhone headphones are designed to work on iPhones (big surprise there). The Skullcandy headset is likely designed for the iPhone also. Unfortunately, there is no "standard" for how headset controls are supposed to work on smartphones. I know that its pretty difficult to find headsets where all the controls work properly on Android (despite Android now having the majority market share on smartphones). I except the same or much worse for WP7 support.