I get great sound output to an external speaker with my Macbook Pro, however, when I output to headphones the sound diminishes greatly. I have reset PRAM and the sound volume output improves, but only for a short time. Settings all appear to be set properly. I'm thinking there's a hardware problem. This is why: I have a monitor that I connect to my Macbook with an HDMI cable. The monitor has a headphone jack. If I plug my headphones into the monitor's headphone jack the sound output is great. Any thoughts? Thank you.
Could definitely be hardware related. Try testing out the headphone audio in a new "test" user account. If it continues, its most likely hardware. If its still under warranty I would take it into the Genius Bar and have them test / replace the logic board. You can check the warranty status here. https://checkcoverage.apple.com