[op25-dev] OP25 GRC - Almost but not quite working

I have the OP25 GRC demo that Balint provided up and running. Everything seems to working except I'm not getting any audio out of the OP25 block. I'm getting the "four line" output from the dibit output port when there is traffic on the channel, and the autotune output is outputting data. However, no audio. I put a scope on the audio output and it's a flat line at zero, even when the dibit output is "four lines". Any tips on how to further troubleshoot? Thanks, Andy