Hi! That's definitely strange and I don't know of any minimum rate, so I'm not immediately sure what the problem is By "don't seem to play", do you mean that you see a blank player with no waveform visualisation? Or does it show up, but playing fails? (And if that's the case, is there an error in your browser's JavaScript console?)
Under the hood, we use wavesurfer.js to power the audio player and typically stick to the defaults here. So if Wavesurfer can play an audio file, it should work in Prodigy's audio UI.