..not sure where to put this really, but whatever.

REAPER won't pick up any sound at all when I try and record, Audacity does, and it'll play it back and everything, but for some reason REAPER won't.

I went through all the settings and everything is good.. I don't understand why it won't pick any sound up.

Any help?
check your input and output devices in your preferences.

What're you using as your input? Audio interface, direct line in, mic?
Options > Preferences, make sure that the stuff in the Audio > Device page is correct.

Hit the small R on the track you're trying to record, then check if the meter is picking up something. If not use the drop-down box on top of the meter to select the right input.

Hit the record button. Finito.
Yeah, the meter is picking up something now, but it won't play anything back.

There's no sound.

Everything is set right though.
..and yes, my volume is up and nothing is muted.
Again check the outs in the audio > device page, check speakers on, interface on etc....

Actually the most stupid thing happened to me a few minutes ago - forgot that the sound was muted Not something you usually think of...
the ASIO driver is kind of picky about having complete access to your audio card so I usually have to close all other programs for it work (Winamp, open Youtube pages, etc...)