The 'fix' for sound to work well as a media centre is to use HDMI :) HDMI sound is excellent but the built in analog sound drivers crackle badly every time they are started and stopped. AFAIK there has not been a fix for this and iti s supposed to be quite hard to ramp up the Alsa volume on starting a sound stream.
Normally to change soundcards in linux you can use Alsamixer - I haven't got an external soundcard for my shack Pi yet (the other PI is indeed a media centre using HDMI for both sound and video) so I haven't tried this. I will be interested in your results when you get an external USB sound card working to know whether the Alsa crackling problem is only on the built-in card or whether it occurs on USB cards too. I've been playing about with speech synth and the startup-stop noise is annoying.