I've got the same issue with my game project, and sometimes music files, sometimes ajax loads of text files, and sometimes spriter files fail to load. Just to reduce the potential for errors I removed the use of ajax from my project, but the music and spriter files still fail to load most of the time.
The frustrating thing is I've spent the last hour trying to make the simplest possible example project which reproduces the problem, and it loads several music files and several ajax loads of text files and I've added a spriter object and so far it's loading everything perfectly. My best guess is it's based on a threshold.. that at some point it tries to load too much at once and that causes failure... at the moment all I can think of doing is continuing to add big memory eating sound and graphics files to the project to see if it at some point the load errors kick in.
The only other thing I can think of is we should compare our projects to see if they have anything else in common.