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.
Soundflower is an application that allows you to control where you send the audio from your computer. You can redirect your system audio in to Soundflower and then send it somewhere else. It’s designed for doing a screencast with a voiceover. You can capture the audio coming into your Mac from your external microphone and redirect it into your screen recording app via Soundflower.
But you can also use it the enter way round – to capture the audio going out of your Macintosh and send it somewhere else. For our example we are going to capture all the audio that is coming out of our Macintosh computer and then simply send it on to the USB sound device. Doing this, with Soundflower in the middle, will allow you to adjust the volume controls.
You can download Soundflower from here and install it: https://github.com/mattingalls/Soundflower
Soundflower appears in your sound preferences as a new audio device like in the screen shot above.
Go to your “sound” reference panel and select “Soundflower” as your output device.
This will stop all the sound coming out of your Macintosh computer because it is now all going to Soundflower.
Now install another application called “SoundflowerBed” from here: https://github.com/mLupine/SoundflowerBed/releases
This will give you a new menu item like the picture above. Now select your USB device as the sound output. In the example above I have selected my Scarlet 2i2 as the output device. When you do this you will hear the audio again.
What you are doing is directing the audio output from your Macintosh computer into Soundflower, where the volume can be adjusted, and then you are directing it from Soundflower to your USB output device.