No, you dont need one.
The motherboard has onboard sound that is relatively decent on most motherboards, and the majority of users use 3.5 or USB interface heaphones/sets.
Unless you are planning some 7.1 speakers, or high end audiophile headphones, you wont need one.
EDIT: In regards to the A50s specifically, you can use them via the USB interface, but do note the balance control on your mixamp will not function.