Looking for an app to amplify audio from YouTube
Personally, when I came across a YouTube video with low audio output, I use an app called VLC player and its audio to boost the signal.

The VLC player is free.

Just copy the YouTube video link in the VLC player, then use its Audio to modify it to your liking.

It also comes with a very handy Audio Mixer, allowing you to set up full range of audio output.



VLC also allows you to synchronize the Audio-Video playback and more.

For example, while you're watching the video, just press the J or K keys to move the audio back or forward 50 milliseconds (or more), respectively.