Firstly wanted to say how impressed I am with your project. I'm a professional developer and have worked on open source projects, everything you've done is very nice and hardware is affordable. HifiBerryOS install was slick and easy. All great stuff!
I am having the same fast playback issue mentioned in this post. Unfortunately I can't offer much more info. I have tried using a second Raspberry Pi 3B to rule out something specific to that board but no luck. Let me know if you want me to provide any more debugging?