U3419W, Firestick, cannot adjust volume on headphones

I have a Firestick connected via HDMI to my Dell U3419W Monitor. I am using headphones connected to the Audio Out Port on the monitor. The volume is quite loud on the headphones and i cannot reduce the volume. I see no options on my Firestick to reduce or set a volume level on the device. On the monitor itself, i can access the volume and even if i reduce this to 0, the volume on the headphones does not change. If i use no headphones, I can control the volume on the monitor without issue using the monitor menu system. It's only when connected headphones are used, i cannot change the level of the volume. Can anyone help? I am not using any computer connected to the monitor and there is no other device connected to the monitor, only the Firestick connected to the HDMI port.