I ran into this exact problem when putting linux on a late 2006 imac. I ended up using the mac specific image that ubuntu had available at the time which (based on later research) removed efi and relied on the machine's bios emulation. I tried windows 8, and then linux mint first but both didn't output sound. (I think mint defaulted to optical out with no way to change it)
Booted up into a 64 bit OS just fine after that.