I have the same Problem. Tested it with an old Xbox and a Playstation. There is no Audio when using the Retrotink 2x on the following 4 different devices:
TV Sony KDL-46W905A
TV Samsung LE40M86BDX
Projector Benq W1070
AVR Denon AVR-X2100W
Mine is model nummer 3.1, manufactured 09/2018 and it has been delivered on 02.11.2018.
Do I really have that bad luck with the compatibility?
As it turned out my first RETROTINK 2X had a defect and Matt immediately sent me a perfectly working unit.
Many thanks again to Matt for the quick service a few months ago.