I have the exact same issue surfacing. I have an Intel NUC that has OpenELEC installed on it. I also have a Pioneer VSX-23 that i have the NUC connected to. I have the appropriate audio settings selected within OpenELEC, and I can see video. But when I attempt to play something no audio comes through. It is in passthrough on OpenELEC and the receiver can see it is receiving a signal.
When i switch the receiver to pass-through mode, rather than AMP mode audio plays on my TV. This seems to be something specific with the Intel NUC. I can't figure out a solution!!