I've come here with a similar problem to the OP's - I had no sound while the recording laptop was muted during my sleep.
That laptop (Clevo W230SS) only has the deplorable Via HD Audio sound card, no StereoMix - in the past I've noticed that Bandicam set the sound at "Vista-something" and I didn't have a muting problem, but now it's set to DefaultDevice and Vista is not an option - could a Windows Update be to blame?
However, having read the replies I still cannot understand why I had no sound - here is video showing various methods of muting and the results for each.
(Sorry that this Florida forest sounds more like an industrial estate! AND! Sorry if there are some over-long "waits", but I had to edit in MovieMaker so's to add text, and can only cut at key frames in that program.)
Muting the browser is the only cause of muted recording, but I wouldn't know how to mute a browser without getting up Volume Mixer
, which I never do - I mute either via the bottom-right speaker icon or the Fn key for that particular laptop, neither of which muted the recording during my experiment.