Here's a Report from my weekend testings:
The good news is that the current Version of MediaMonkey is quite stable. Not a single crash over the weekend.
The thing is that the app cannot change the tracks while the Surface is in standby. The current playing track is finished and then the music just stops. When you unlock the Surface the music starts playing again immediately, without pressing any button in the MediaMonkey app. So it's quite obvious that something is wrong with the app running while the Surface is in standby.
What I did: I just turned off the standby in the energy Settings and the Music was playing as I desired it.
The weird thing is that, like Arnold already mentionend, the playback is working good when adding music from the desktop. There are no standby issues. Unfortunately adding music from the desktop is no option for me since I tend to just queue 5 or 6 Albums and let the music play in the background. When that playlist is near its end, I add another 5 to 6 Albums.
This works good with the new version of the app although I hope they address the standby issue...