You can set the headphones level from the "project settings" tab in the IDE. If you want to run it from the command line, then you would have to do something like
/root/Bela/projects/yourProjectName/yourProjectName --hp-level -2
if your program is already built, or check here for more advanced instructions: https://github.com/BelaPlatform/Bela/wiki/Interact-with-Bela-using-the-Bela-scripts