if you are getting no interface sounds at all that would indicate a problem.
But, if you only get no interface sounds while playing DTS audio, I think that is to be expected since the digital stream is being passed directly to your amp and nothing can be added. Same problem with DTS and AC3 movies if you try to use TTS or otherwise generate sounds from your PC. The amp will be locked onto the DTS stream and nothing else will play.
I'm no expert on this, it is just based on my recent experience with my Onkyo 5.1 receiver and XBMC.