You can usually adjust the audio level of your Macintosh computer using the sound slider bar and also using the volume keys on your keyboard. But if you plug in a USB audio device (Focusrite, Presonus, Behringer, Rode etc) the built in Audio controls are disabled. This is so that the full volume can be sent to the device digitally which is important for professional audio applications.
But if you are just using your USB just to listen to music it is possible to adjust the volume for your own listening comfort. Here’s how to do it using a free audio utility called ‘Soundflower.