An alternate way to control the volume is one from the terminal. For example:
sudo osascript -e "set Volume 3.5"
This will set the volume to 50% (the scale is 0-7 using the terminal).
You can use whole numbers or decimal numbers. A higher or lower number will make the volume go up and down in intensity (again 0-7).
This will put your volume at super quiet:
sudo osascript -e "set Volume 0.1"
This will put the volume at an even quieter setting:
sudo osascript -e "set Volume 0.01"
As @timothymh stated, for 1/4 (0.25) increment control without the terminal, you can hold Shift + Option while using the F11 or F12 keys to adjust the volume by 1/4 (0.25) increments.
Using the keyboard will only give you 64 adjustments of volume control
16*4 = 64
Using the terminal you can achieve 700 options for volume control
7*100 = 700