Agora vejo que o texto a seguir no meu arquivo de configuração do i3 faz com que as teclas de mídia e o brilho da tela funcionem.
Controles de áudio por pulso
bindsym XF86AudioRaiseVolume exec pactl set-sink-volume @DEFAULT_SINK@ +2%; exec pactl set-sink-mute @DEFAULT_SINK@ 0
bindsym XF86AudioLowerVolume exec pactl set-sink-volume @DEFAULT_SINK@ -2%; exec pactl set-sink-mute @DEFAULT_SINK@ 0
bindsym XF86AudioMute exec pactl set-sink-mute @DEFAULT_SINK@ toggle
Controles do media player
bindsym XF86AudioPlay exec playerctl play-pause
bindsym XF86AudioNext exec playerctl next
bindsym XF86AudioPrev exec playerctl previous
Controles de brilho Sreen
bindsym XF86MonBrightnessUp exec xbacklight -inc 20 # increase screen brightness
bindsym XF86MonBrightnessDown exec xbacklight -dec 20 # decrease screen brightness