Volume Commander от Ruinwolf

Volume Commander boosts any tab's audio up to 600% of its original level when the source is too quiet — useful for YouTube videos with mastering issues, low-recorded podcasts, voice calls, or any web audio that maxes out at "still too soft." A built-in dynamic-range compressor smooths the boosted signal so loud peaks don't blow out into harsh clipping.

The toolbar popup gives you a single slider (10% – 600%), one-click presets for 100% / 150% / 200% / 300% / 400%, and a Reset button. The display color shifts from cyan (normal) to green (boosted) to gold (high) to red (extreme) so you always know how aggressive you're running.