As others have said, you don't need a soud card. The only reason to get one would be if you have a 5.1/7.1 setup that you want to use thru optical or coaxial as mobos usually don't support Dolby Digital/DTS. If you plan on using the analog connection or only have 2.1 speakers you'll be be fine with onboard sound.
The ath-m50s don't need an AMP so they'll be fine with the onboard audio. --- i7-920 @ 3.6 // 770 GTX // 12GB G.Skill Ram // My Setup: https://imgur.com/a/2Gd8M FiiO e9+17 // AKG Q701 + M50 // Deck 82 // G27 // Qanba Q1 // PS3/360