Alsa is standard for slackware. I haven't tried openbsd (yet). As for slackware I haven't had any problems lately. When alsa was first introduced, there were some issues, and i.e. you manually had to start firefox using alsa, or map oss to alsa (or something. I cant really remember how i fixed it), otherwise firefox (Flash plugins) would mess it up by using the soundcard directly instead of using oss-emulation.
Since slackware version 11(?) and up, i can't recall having any problemes with the sound nor alsa.