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EdgeVolume is a free, open-source macOS menu bar utility that lets you slide a finger along your trackpad's edge to adjust system volume, with customizable edge selection and sensitivity. It is currently in early access and was featured on Product Hunt.
SOUND is a macOS app that provides per-application audio equalization, volume control, and speaker selection from the menu bar, addressing the lack of built-in per-app audio settings.
A blog post explains how to adjust a Mac's system volume from the command line using AppleScript and provides a custom bash script for easier control.