Thanks for letting me know! I think this is a rare bug, and I'm pretty sure it's caused by the audio element not properly loading. It's definitely on my todo list!
I will probably make it availble once I've cleaned up the code and made some improvements to the algorithms. Beat detection can be spotty on constantly loud songs.
I'm not sure how milkshake is generating it's visualizations, but with Vizrify I was playing with using the new WebAudio API (which will be coming to firefox in a while) to access html5 audio element FFT data directly. It also opens up the avenue to do other cool audio transformations that are difficult to do otherwise.