checked out the linked gentoo forum discussion on this topic and as of today it still doesn't seem solved. i thought i had this same "bug" after a clean install of sabayon 5 kde (x86), but i found a way around it. both amarok and flash are playing after i raised the PCM volume in KMix. it was turned all the way down by default, but doesn't appear to be having problems.
not an expert at all, so i can't help any further if this doesn't work, but it might be worth a try! good luck!