Setting the correct ALSA device.

[Tim Curtis]

Very odd. "Nothing selected" suggests some breakage. 

To troubleshoot try the following: 

1. Unplug USB DAC
2. Open Audio Config and set I2S audio device to "None"
3. Reboot
4. Open MPD Config
5. Verify that Audio device = "On-board audio device"
6. Plug in USB DAC
7. Close then Reopen MPD Config
8. Verify that "USB audio device" has been added to the Audio device dropdown