First, there's no point in directly modifying privileges on the /dev directories, since those are changed by the system on login. So any change you make will only last until next time the first user logs in. In addition, it doesn't directly involve artsd, or alsa, or any of that.
Instead, this has to do with the way Mandriva configures sound support for multiple logged-in users.
The sound devices get owned by whoever logs in first. This is inconvenient for multiple local logins, but prevents annoyance when remote users can login.
The behavior is governed by a script /etc/security/console.perms.d/50-default.perms. But there is no need to modify that. The MDV crew set it up in a flexible way.
There is a system group called audio. To let multiple logged-in users share sound, just add your non-system users to this group. In Mandriva, you do this with MCC (also called "Configure Your Computer") under System -> (blah, blah) Users on Your System. Be sure to uncheck "Filter System Users" if you don't see the audio group listed.s
At least, that works for me with MDV2007.
It'll also work even if some logins are KDE and some aren't. It's to do with how the system handles user login rather than with the particular session type.