You most likely won't have luck with AV receiver. There have been reports with same issue- that 5.1 can't be recognised by a HDMI receiver.
For this you would need a single track flagged as 5.1. Solution could be some HDMI box with ability to convert mono tracks to single 5.1 track. I've pointed to such a (possible) devices in other thread:
viewtopic.php?f=21&t=137944Some BM devices have now a setting to present a proper 5.1 stream on HDMI.
You would just have to create file with correct order. I assume it takes 1-6 channels and maps them according to this setting. Whole thing is bit convoluted, but this is due to mixing pro/broadcast equipment with home one.
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Other solution would be to use some pro player which allows to map channels and send them over BM card to your receiver. This is probably possible even with ffmpeg/ffplay. Question is: does BM SDK allow to get 5.1 flagged track on HDMI output for their cards? It's possible that you can't do such a thing at all.