Hi Mike, I just saw this after I sent my other e-mail.
The volatges match the spec sheets, i.e. 5v for the Cubietruck and 3.3v for the Wandboard at VCC whether audio is playing or not.
Yes, this might be the issue.
So unless both my receivers have a problem with only 3.3v I'm at a loss. I can't find anything on the limited specs for the receivers relating to this.
I think you can just test your receiver(s) with another 3.3v source and if you see the same problem then they probably need 5v levels.
I guess I'll just sideline the Wandboards and use proper kit instead.
If it turns out that the voltage level is really the problem and you need to use those receivers I guess this would be the case. I am not electrical but you might be able to build a small adaptor to change the voltage levels on the spdif output. This would be another solution.
Regards
Sinan Akman