I know I'm not the only one having this problem, but there's still no solution. I posted about it in Microsoft Windows 8 forum and was send to post it here again.
When I'm playing locally stored music via the XBox Music App on Windows 8.1 (x64 Version + RT), I sometimes hear advertisements between two songs, just if I was streaming the music. But it is stored locally on the PC I'm using to play it. The exactly same issue is described here: http://answers.microsoft.com/en-us/windows/forum/windows_rt-pictures/does-xbox-music-stream-music-even-if-it-is-local/254c6ab5-2734-46f5-980d-c33176bf02b9. The given solution is to turn off cloud syncing, which is no solution for me as I'm (independently from this problem) using that one to stream the music to my surface 2. Another solution I found to work is to switch the displayed songs from "on this PC" (what is initially selected) to "all music" and then back to "on this PC". After that, I don't hear any advertisements anymore. But to do so every time I'm running the music app is really annoying. So, is there any fix for this Problem?