In simple word external sound cards usually provide better sound quality over the built in one of a mobo and a trained ear will certainly be able to catch difference.But u need to have good quality speaker set or hp to appreciate the difference.As for hp, good sc tends to have good built in hp amp which can be usefull to run high impedance hp.But all these crispy quality will be benefecial if u have good quality hp/speaker.There is lots of sound card in the market from cheap chinese one to T;HD.Now it depends on ur budget.