This script adds a volume slider to the timeline tool bar.
Extract the folder to 'SourceFilmmaker\game\workshop\scripts\sfm\mainmenu' before starting sfm
You'll find the script under the script menu. There is also an option to add it at startup
the only issue with this slider is that it will overlap some tool buttons in the graph editor if you shrink the timeline window too much
feel free to modify the script for your own use if you find any bugs or have suggestions message me in the comments below.