Sound output keeps reverting to Internal Speakers after boot

Every time I reboot my Mac Pro, the sound output resets to the Internal Speaker. Then I have to go to System Preferences and switch it back to Line Out so I can use my speaker system. But it doesn't stick, the next time the computer restarts, it's back to the Internal Speaker. Any ideas how to fix this?

Here are my system specs:
Early 2008 Mac Pro 3,1
Mountain Lion 10.8.4
Processor 2 x 2.8 GHz Quad-Core Intel Xeon
Memory 16 GB 800 MHz DDR2 FB-DIMM
Graphics NVIDIA Quadro 4000 2048 MB

Any help would be appreciated!