If you believe the issue is with your Macbook Pro you might want to use the Apple online support center. Ask "Why might my Macbook Pro suddenly lose volume?" If that doesn't work give your local Genius Bar a shot. I have had great outcomes with the "geniuses". Good Luck!
Volume suddenly dropped via my MacBook Pro
Help! I have a MacBook Pro in bought in 2016, using it with a Bricasti M1 DAC. While listening, the volume suddenly dramatically dropped. The rest of the system works fine, and volume is normal using the a CD through the M1. Thoughts on how to restore the normal volume level while using my MacBook Pro? I use J River media center. Thanks!