I suggest that bandicam has a "test" button that allows users to check the microphone from within the program.
Windows sound test is good also. But if 3rd party program(s) are picking up sound ok, then it helps isolate if it is bandicam-specific or windows.
Snagit, Camtasia, Skype, etc have an ability to quickly check if the microphone is picking up sound properly. I'll usually run a test there first if I have problems....although normally I do not do sound with screen recording. I narrate later.
Also, it could be possible that an app is using the microphone. But Bandicam should be able to check this internally and report to the user that it is unable to take control of the microphone.
Another trick is to go to windows services and restart the windows audio endpoint service. This will also restart the windows audio service.
If windows suddenly stops playing audio (normally sometimes after waking up from hibernation), then you can restart windows audio service and that usually fixes things.