adjust volume of audio track

[KevinYork]

The fewer issues I have to read, the more new features I will have time to implement, so I ask that you please try these things first

• Check out all the menus and keyboard shortcuts. Maybe the feature already exists?
• Try with the newest version from GitHub
• Read the documentation and FAQ, Known issues, Troubleshooting to see if the feature already exists
• See if someone already suggested this under Issues or Discussions

Description

Adjust volume of the audio track. [I realize that is a lossy op; but...]

I am suggesting:

Changing the volume of an audio track:
	ffmpeg -i input.mp4 -af "volume=0.5" -c:v copy output.mp4
	ffmpeg -i input.mp3 -af "volume=3dB" -c:v copy output.mp3

[mifi]

• Read the documentation and FAQ, Known issues, Troubleshooting to see if the feature already exists

#372