http://science.nasa.gov/science-news/science-at-nasa/2003/09sep_blackholesounds/ it says 57 octaves below middle c. not exactly sure the freq but its lowwww.
theres just one part im confused on. space is a vacuum. sound cant occur in a vacuum. how can there be sound, however low and inaudible it may be, its still sound, if its a physical impossibility?