Nvidia GTX550 running driver version 310.19, AMD Phenom 9550 quad core, 4 GB RAM, Windows 7 64 bit. View distance set at 3200m, most settings on High.
I have seen a lightning flash exactly once at the start of a MP mission someone else set up. No sound. Nothing before or since. All my tests in the editor (100% lightning, 98%, 95%) show no lightning at all.