Examine the file with MediaInfo to determine the type of audio that is in the video.
Open the file with AviDemux and let it index the file if necessary. Once AviDemux has done that, go to Audio (at the top of the main window in AvidDemux)–>Save. This opens a window where you can select where to put the audio, and allows you to name it. Make sure to name the audio file with the correct extension. If it is an mp3 file, use .mp3. If AC3, use .ac3 and so forth.
Now, Audacity may very well import the audio without having to use either of these tools. Make sure to get the ffmpeg plug-in at the Audacity site, then try to open the video file with Audacity.
In any case, once you have the audio file and have worked with it as much as you think possible in Audacity, export it (to save) and then replace the original audio with this one. You can do that with AviDemux…probably Nero too, but I quit using Nero a long time ago and cannot advise you on it.
To replace the audio stream in AviDemux, keep the Video on Copy in the main window of AviDemux, put the Audio (just under the Video button) on the correct format for whatever your revised audio stream is, then click on the other Audio button at the top of the screen in AviDemux. Click on Main Track, then select an external source for the audio, and direct it to the place you have the modified audio stream.
Now, make certain to set the Format on the main window in AviDemux to MP4.
At this point you are ready to save everything. Click on File–>Save–>Save Video. This opens a window where you make a title for the new video you are putting together. Be certain to give it a name with an .mp4 extension.