Help with editing video sound

Audacity does not edit video. You can install FFmpeg then let Audacity extract the audio from the video to its internal lossless audio format. Or you can create a separate WAV file in Bandicam as you have done and import that WAV into Audacity.

Then replace the audio in the video using a video editor such as listed at AVIdemux should replace the audio fine without having to re-encode the video. Re-encoding the video always loses quality because it is not practicable to use lossless video formats. Lossless video is of colossal size and apart from the storage issue, computers will find it hard to play such video smoothly.

Some video editors will let you apply filters to the audio track directly but there is no noise removal in AVIdemux.