you're right, that's what i exactly did.
i just used current kubuntu 7.04 live-cd and everything worked fine.
i'm not really sure if the kernel or alsa caused the problem but for me it seems to be the kernel. the alsa version on kubuntu 7.04 is the same as in openSuse 10.2 (i think its alsa 1.0.13).
the kernel kubuntu 7.04 uses should be 2.6.20-14 as far as i know.
openSuse 10.2 uses 18.104.22.168-34, that's the problem.
it was a good point, thanks.