Is it possible to run one artsd for multiple users (I have only one soundcard device)?
Yes, but AFAIK only one can use it at a time which is fine for a desktop system where you switch from one user to another.
To have it switch between users you should set: "Auto-suspend if idle after" to "1 seconds" in the "General" tab in the: "Sound System" KCM.
Be sure that you have the permissions for the sound devices set correctly and that PAM doesn't screw things up.
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