All you've done here is take the 'hidden in pi' argument and customized it. If you are only making a sample of random mp3s you have a statistically zero chance of infringing on anything. If you make a thorough set of X-length mp3s then the actual infringement is in the url, because a datastore that has every number is equivalent to one that has no numbers--it's really just an encryption algorithm between url data and mp3 data.