I downloaded a new version of Adobe Flash Player and now my sound level is either on maximum or off and I can't adjust it.

Flash Player still exists?

Anyway, I have the same problem, didn't do anything, it just started today. Console when I try to change the volume:

Lichess is open source! github.com/ornicar/lila
7jquery-ui.slider.min.js:6 Uncaught TypeError: Cannot read property 'toggleClass' of undefined
at t.<computed>.<computed>._toggleClass (jquery-ui.slider.min.js:6)
at t.<computed>.<computed>._addClass (jquery-ui.slider.min.js:6)
at t.<computed>.<computed>._mouseCapture (jquery-ui.slider.min.js:6)
at t.<computed>.<computed>._mouseCapture (jquery-ui.slider.min.js:6)
at t.<computed>.<computed>._mouseDown (jquery-ui.slider.min.js:6)
at t.<computed>.<computed>._mouseDown (jquery-ui.slider.min.js:6)
at HTMLDivElement.<anonymous> (jquery-ui.slider.min.js:6)
at HTMLDivElement.dispatch (lichess.site.min.js:2)
at HTMLDivElement.g.handle (lichess.site.min.js:2)

Thanks for reporting. Tracked down to github.com/ornicar/lila/commit/e972d5612df11855f39bde57bed3796f02b769b2. Will be fixed shortly.