i fixed it now by using ALSA in OSS emulation mode.
i can't really tell for the SB PCI128 but I think it's rather a generic problem.
i guess you should use ALSA.
I've got PCI128 and I can't really recall the issues with oss but I believe you need to buy their commercial package to have full duplex. I've been running alsa in oss emulation mode and that works fine. Just make sure you get the latest alsa version (at least 0.9.0beta7, well that's what I have) because the earlier had a bug and would break into this annoying digital noise loop after running audio for some time.
HTH
./MiS