I just extracted the AC3 core from the TrueHD file and inserted it into the mkv.

I also have a BD-C5900 and it cannot play TrueHD or DTS-HDMA from either DLNA or from usb HDD. I tried extensions .mkv, .ts & m2ts and still doesn’t work. I am very disappointed as I have an AVR that is capable of decoding HD Audio and neither my Samsung TV nor Blu-ray player can read them.

Thanks anyway for your help Dennis.