actually it wont get called if you dont also have GetPCMRawData exported. I will fix that tonight
Your guess and the following fix did the trick. I'm very much happy with how it works!
I want to state two lesser issues, without pushing for a fix. It may be useful for the stabilization of the api.
1) If two plugins (one c++ (background) and one c# (with visible panel)) are implementing GetPCMRawData, then GetPCMRawData for the c++ one does not get called. I have a c++ bridge plugin, so I found that by accident.
2) You close the player while playing a song with this option selected

. After starting the player again it continues to play, but GetPCMRawData and GetStreamInformation does not get called, unless another song is selected.