I have this issue with my HTPC set-up where the audio quite often crackles; especially in action scenes.
HTPC is home-built computer with this mobo: http://www.asus.com/Motherboards/M2NVM_DVI/
No sound card fitted.
Audio connection is a basic cable from mobo to amplifier Amplifier is a 10 year old Prologic Pioneer (I was hoping to upgrade but the funds were needed elsewhere)
Could the issue be as simple as adding a sound-card with RCA? This was suggested by a mate.
Or am I looking at something more expensive?