I have been using this alarm clock application for my 2012 Macbook Pro for a couple years now and nothing beats it. It has the capability to override the system volume flawlessly and this setting can be toggled on and off in the preferences section of the application. It has been compatible for me for every OS from my current Yosemite all the way back to Mountain Lion.
Mahssi666143777 about Wake Up Time - Alarm Clock