Now you are back to the Wiki article on Exclusive Access:
http://wiki.jriver.com/index.php/Exclusive_accessSee the section titled: "Issues with Disabling Exclusive Access in WASAPI"
In particular, this paragraph:
If you disable Exclusive Mode in WASAPI on Windows, then you must set Media Center's Output Format DSP to resample all audio to match the format you have selected as the Default Format for the audio device. In WASAPI non-Exclusive mode, the Windows Default Format setting now becomes the only output setting that MC can use, and you must resample all other formats to match this format.
That doesn't quite explain why you aren't getting sound out of YouTube etc., but it make the audio chain more complex, and there could be a problem in the configuration of MC DSP and/or the WDM or RealTek default audio settings. With non-exclusive mode the audio chain is something like:
YouTube > Windows Sound > WDM Driver > MC Audio incl. DSP > Windows Sound > RealTek Driver > Windows Sound > Speakers
I don't know how often or where Windows Sound actually kicks in, I'll leave that to the experts, but it could be as bad as that! I'm a little out of my depth in this area. Perhaps some expert in this area can advise.
By the way, I've noticed that WASAPI Exclusive Access turned on plays loader than with it off; why is that?
Because with Exclusive Mode off, Windows Sound can change the audio as it sees fit. So it is at least reducing the volume, perhaps when down mixing, and probably changing the Bitrate and Sampling Frequency.
The disadvantages of non-exclusive mode were the reason I was happy to compromise and effectively lose short Windows sounds, like the Ding, etc. when using the WDM Driver.