I had the same problem with my MacBook Pro 13" with Retina Late 2012 and the HP ENVY 32" Beats Audio monitor. My following workaround might not be a perfect solution for the original poster as they are using two computers with the same monitor. Changing the audio input every time you switch the computer input would be a pain. If you are only using your MacBook with the monitor this should do the trick as you only have to change the audio input once.
1. Using the supplied 3.5mm audio cable that came with the monitor connect the headphone output on your laptop to the analog audio input on the monitor next to the USB ports on back.
2. On the mac go to "System Preferences" under the "Apple" menu and select "Sound". Select the "Output" tab and then choose "Headphones" as your sound output device.
3. On the monitor there are three buttons along the right side. Hit the top button. Hit the top button again to select the "Main Menu". Use the bottom button to scroll down to "Management". Hit the top button to select. Scroll down to "Audio" and select. Scroll down to "Select PC Analog Audio" and select. Hit the top button to "Save and Return". Hit the top button again to "Save and Return" and again to "Exit". You should now have sound control from the volume keys on your keyboard.
Make sure that the volume control on the monitor is not set to mute. The volume control wheel is located underneath at the bottom left of the monitor. Pushing it in will mute and un-mute the sound output. I have mine set to level 70.
I hope this works for everyone.
- RebelPhil
