No direct keyboard shortcut.
Keyboard shortcut to system preferences fn+control+F2, then arrow down to system preferences hit return.
You can write a shortcut to get to sound, then use the slider.
Syst prefs/keyboard/Keyboard Shortcuts tab, click on Application Shortcuts in the left, + symbol in Application: select System Preferences.app MenuTitle: type in Sound Keyboard Shortcut: type in command7 (this is just an example you can use what you want)
The keystrokes would be:
fn+control+F2, Down Arrow (5 times to system prefs) Return, comannd7, then use mouse to move slider.
Not a very big improvement.
Have you tried resetting the PRAM?
Power off, power back on holding the following keys: option+command+p+r before the start chime, continue to hold till you hear the start chime 2 times then release. See if that helps.
You can also try the following to see if it helps.
From the Finder Menu click GO/Go To Folder or SHIFT+COMMAND+G type or copy paste in the following:
/Library/Preferences/Audio/com.apple.audio.SystemSettings.plist
Drag that file to the desktop, reboot and see if the prolem persists. If it does not you can trash the plist from the desktop, if no improvement drag the file back where it was (you can get using the same path as above) it will ask to authenticate, click replace and enter your admin password.
You can also try the same proceedure with the path
/Library/Preferences/Audio/com.apple.audio.DeviceSettings.plist
