Using ffmpeg to increase audio within WAV

I have a WAV file created using my Nikon camera but I was too far away from the subject, so the audio is very low. How can I increase it?

Each ffmpeg command I have run seems to only extract the audio and leave me with a new audio file without the video, but it also doesn’t seem to actually increase the audio either.

$ ffmpeg -i sigma.mov -af "volume=5dB" -c:v copy -c:a aac -b:a 192k sigma-louder.wav

This leaves me only with a WAV audio stream:

$ file sigma-louder.wav 
sigma-louder.wav: RIFF (little-endian) data, WAVE audio, stereo 48000 Hz

I’ve also tried variations like this:

$ ffmpeg -i sigma.mov -filter:a "volume=1.5" sigma-louder.wav

This also just leaves me with an audio file, but it also doesn’t appear to make it any louder.