AudioBoost by NPlusOne

A Web Extension that inserts a Web Audio API GainNode and DynamicsCompressor node to increase the audio level of videos that are too quiet, while limiting the maximum volume of those videos.

This is an enhanced version of https://github.com/padenot/up-to-eleven: the input gain for the gain section, and the threshold and ratio for the compressor are now controlled by sliders in a pop up window.
Features

There is an enable/disable button in the pop up window that allows you to turn the extension on and off.

There is a VU meter to see the audio levels.

The Gain controls amount of gain applied before the compressor You can set this to 1 and there will be no additional gain, only compression.

The Threshold determines the volume levels above which the compressor will affect. All the way to the right and the compressor will do nothing. All the way to the left and the compressor will force all audio to the same volume(or try to). Somewhere about -20 is a good place to start.

Ratio is how much the compressor "compresses" the audio. To make everything the same volume, increase the ratio above 10. For a subtle effect, set the ratio to a low setting like 2 or 3.