I seem to recall that you need to turn recording off for the track you want to hear playback on.
The button the top of or above that track or channel must be green (playback) not red (record).
if that's not it, check your signal path levels just in case.
Although, if you create a new project these should be set at the default levels already.
If you don't have the manual, you should download the PDF version of it from Zoom's site.
it's a bit of a dense read the first time, but as you play with the MRS-8 and refer to the manual, it will make more sense and become easier to find what you looking for.http://www.zoom.co.jp/archive/English_Manual/E_MRS8.pdf
also follow Surfercook's link and get the Card Manager.
you'll need that later if/when you start editing and/or mixing on a computer.
there are 2 version, one for the PC and one for the Mac, so be sure to get the one appropriate for you computer.