PulseAudio volume resets to 100% every reboot

I configured my RPI3 for Network Audio Receiving via PulseAudio according to the wiki and everything works great.

Additionally, I set volume to 50%: pactl set-sink-volume alsa_output.0.stereo-fallback 50%

However, after the system restarts the volume resets back to 100% which nearly killed my dog. I tried adding the pactl command into /storage/.config/autostart.sh but it has no effect.

How can I set the volume to 50% by default?

I am not sure what you mean. Like I said I running pactl set-sink-volume alsa_output.0.stereo-fallback 50% will set the volume to 50% but it does not persist over system reboot and adding that command into autostart.sh has no effect.

Running pactl set-sink-volume 50% without a sink name/index doesn't work:

You have to specify a sink name/index and a volume

I have searched countless websites and wikis and setting flat-volumes = no should solve my problem. I followed the advice above but unfortunately after I reboot the system the volume is reset to 100% again every time.