Please refer to the "Record your voice with the "Two Sound Mixing" option.(Windows Vista/7/8/10)" section of the linked page and try recording again.
https://www.bandicam.com/faqs/how_to_re ... icrophone/
Please note that Windows allows programs to use only one audio device at one time. If the microphone device is used by another program, you will see the "Failed to start audio capture" error message.