Sound of the big bang

http://www.astro.virginia.edu/~dmw8f/sounds/cdromfiles/index.php
(ref from this week's new scientist)

BTW even today there is sound in interstellar space.
Just that it is normally very low frequency indeed. The lower the
frequency then the fewer the number of atoms you need
in order for a sound wave to propogate. But the
lower the temperature, then also the fewer
the atoms you need. So in the denser interstellar
gas clouds, quite high frequencies may be possible,
up to 40 Khz.

http://www.astronomycafe.net/qadir/q2709.html

Robert