mmm... I'm not sure about devfs or udev. I'm using slackware 9.2. I have write-read-execute permisions on /dev/audio and /dev/dsp (I'm not in my box right now... have no internet on it.. so the spelling may not be exact). It doesn´t work even if I'm root.
What I first tried was to generate a signal, using octave, trougth the speakers. I had some problems, until I put the read-read-execute permisions on /dev/dsp. but have no idea about what other permisions I must have.