Have you enabled Bitstreaming? If so, try turning it off. If not, try turning it on.
Pretty certain no restarting Media Center is necessary, but do make sure you have stopped all playback vs. just paused it.
Also check Exclusive Mode on your selected audio device in Media Center. Maybe try toggling that.
Check DSP settings - are you perhaps converting anything to another format that might not work?
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Also - even thought your default device says AVM 70, I expect you are using your video card's sound drivers to access the AVM 70. For me, it says "marantz AVR" and the next line says "nVidia High Definition Audio" as I use an nVidia video card. Maybe poke around those settings