I don't do Macs, but I'll take a stab in the dark until someone else has a better idea...
We'll assume that you're not newbie enough to have simply changed the tempo
Next newbie thing, but less newbie.. check that the playback rate (in the same general area) is still set to 1.0.
Outside that, I would wonder though if your sample rate in Reaper is not matching whatever your audio interface / sound card is either set to or capable of. So perhaps check that.. check in Preferences->Audio->Devices. If you didn't tick the box that says to request a rate, it should just default to whatever the driver of the card has set the sample rate to. Even if you check that though and pick something, it should still work, afaik.. but maybe not if you have some oddball value in there.
Lastly, if you have multiple audio devices, make sure you're using the right one, and make sure that it is using the latest drivers. That said, sometimes the latest driver isn't the best driver, so maybe share what audio interface you use, in case there is a known issue with it. You could also try an older driver even (if you have the newest installed), just in case.