can't set the device for system (event) sounds in pavucontrol

[patrakov@…]

In pavucontrol, there is a button near each stream's volume slider that allows to move the stream to a different device. However, the "system sounds" slider doesn't have this button near it. Thus, it is impossible to configure my preference "all event sounds should go through the PC speaker (alsa_output.platform-pcspkr.analog-mono)".