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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.