Not compatible with Greasemonkey 4.1.
Please use Violentmonkey or Tampermonkey for this script.
This is a revised version of https://greasyfork.org/en/scripts/10197-set-html5-video-volume
that handles both audio and video players.
Note #1: The script does not adjust the position of the volume slider on custom control bars (e.g., YouTube) but it still should have modified the volume in the player itself.
Note #5: If the default value of 50% is too high or low as your starting point, you can edit the script to change 0.5 to your choice of values.