Ok...
The way I would do this is....
Get Cakewalk, it allows for video import (not sure if you can actually edit video there)
It will create audio in separate track. I would edit that audio / insert FX and export that video with edited audio.
Another way is.... Follow Cakewalk steps, but export just audio.

In your video editor (one open source editor I tried is Shotcut)
Import your original video with original audio. Create new audio track. Import that mp3 exported from Cakewalk. Position match to original audio, mute original audio. Slice and cut your video. Export.