You either need a 11pin
ExtUSB to 3.5mm adapter, or OEM headphones designed to work with HTC's propriety
ExtUSB plug. Most of the common USB headphone you find in stores are designed to work with Motorola phones, and are not compatible with HTC products
The 11pin
ExtUSB to 3.5mm adapter looks like
this and one end plugs into the phone and the other end you plug your regular headphones into.