Evenwave

You know when a video only plays in your left ear? Evenwave fixes that. It listens to whatever is playing, and if one channel goes silent for a few seconds it sends the other one to both ears. A small note pops up on the page so you know it kicked in. When the audio goes back to normal stereo, it gets out of the way. The popup shows live left and right meters and lets you pick a mode: Automatic, Mono, force left, force right, or Off. It works on the major video and music sites out of the box, YouTube included. For any other site, open the popup there and hit "Turn on for this site". It only gets access to the sites you choose. Copy-protected players can't be adjusted, since browsers don't let extensions touch that audio. Evenwave will tell you rather than pretend. No accounts, no tracking, nothing leaves your browser. The only thing it stores is which mode you picked.