trigger in rules a beep sound on user side (current user is working)
In order to send a message to users other than the current user, you will need to do some additional processing. drupal_set_message sets a message in the current user session. It does not set a message in another user's session.

Some possible approaches include a push or pull notification type system.
For example, the server can push updates to the client using something interesting like a Service Worker (advanced). Alternatively, the client can poll the server periodically, via ajax type request to get message updates.