So now the sound server just points all audio to /dev/null which obviously is undesirable.

How may I fix this?

Have you tried doing "chmod 755 /dev/dsp"?

Yes, I tried that today. Thank you.
After rebooting, ls -l /dev/dsp shows that the permissions have been reverted to crw-rw----. What does the c stand for and why hasn't the mode been kept?

Even if I chmod 755 /dev/dsp and then restart KDE, the message pops up again with "/dev/dsp can't be accessed".

And I suppose this may be a related problem (with file perms and devnodes and things: I get a warning once I log in saying "You are not permitted to connect to the powersave daemon via DBUS. Please check you DBUS configuration and installation". What implications does that have on the use of my laptop and what may I do to fix it? (every time I start KDE the little system tray thing with the battery appears. If I useKDE twice then I get two of these.)

I'm on debian etch by the way, not slackware 12 as my profile says.

The c just means it's a character device. As for the permissions, it sounds like the device is being recreated automatically, for example by udev on boot. What's the group owner of the dsp device? I'm not familiar with the way debian does stuff, but maybe you need your user added to the 'sound' group or similar in order to access the soundcard. For example, gentoo has an 'audio' group.

Good luck.