I am looking for a native Linux software that can increase the audio volume of a video file.

To increase the volume by a specific decibel amount (e.g., +10dB) while copying the video stream:

ffmpeg -i input.mp4 -vcodec copy -af "volume=10dB" output.mp4

To increase the volume by a linear multiplier (e.g., 2x louder):

ffmpeg -i input.mp4 -vcodec copy -af "volume=2.0" output.mp4