I also have the M-freebox and observe the same behavior. Works fine with TuneBlade, but no audio output (at first) when using SWYH. I tried SWYH with a different DLNA renderer (running on Android) and that works fine.
After some tinkering, I was able to get some output from the M-freebox by adjusting the streaming options in SWYH (MP3 instead of LPCM, bitrate, KHz), but the playback was very choppy.
It appears this issue could be fixed by GGMM, or SWYH, or both. Best to also send a bug report to GGMM since their firmware could use improvement – there are other apps that don’t run smoothly with their device.