DFRduino Player module: Ripple in sound problem

Hi there, thanks for providing a great sound module. A pitty there is no official .Net Gadgeteer driver... But I managed to mock something up to have it playing sound in my speaker.

The problem is that I am hearing a ripple, especially at low volumes. I have tried several different sound files from different sources, and it seems to me that the ripple frequency is changing with the sample rate??

Can you help here?