I needed to upgrade to 10.7.5 to get my graphics card to be compatible, but now the sound doesn't work. In system preferences is says there is no built in sound and doesn't recognize any external sound devices either. I thought about upgrading to Mountain Lion, but my Mac Pro is 2007, so it won't work. I have looked at lots of threads and tried various things. I have resetted all the pram and smc settings, I have gone into system preferences. It is happening to a lot of people but here seems to be no answer. I have a Mac Pro 2007 Intel, 2x Dual Core Xeon processor, with GTX 560 graphics card, 250gb SSD, 3TB in sata drives, 24gb ram. I really need sound for my work as a video editor, I think I might loose my latest job if I don't get it fixed soon.