Found it...

Microsoft is currently fixing a recurring YouTube audio issue causing muted or lowered volume while watching content.

安啦~ 過段時間就會自己好了

我寫得太快, 不一定是Google搞的, 但可以說是微軟的確有察覺到這個問題.
(排除掉陰謀論的部分, 只講實際發生的情況)