Here's a video I made comparing the Canon 7D (in live-view quiet mode 2) to the Olympus E-PL1. The latter is noticeably louder despite the lack of a mirror. This seems pretty strange to me, because a DSLR in "quiet mode" basically just takes the mirror out of the equation...and if a DSLR can be that quiet, why can't a mirrorless camera?
http://www.youtube.com/watch?v=z_wvNBrn_gE
http://www.youtube.com/watch?v=z_wvNBrn_gE