Ctrl-Alt-X opens Reaper's Media Explorer. There, make sure 'Tempo match on' is set in the drop-down window, and also the checkbox 'start on bar' and 'auto play' are ticked. (To the right is a volume bar which you can set later) I suggest you also enable the loop button next to the stop and play controls. Now you can just hover the mouse cursor over an drum loop (wav, aiff etc) and it will play in sync to the project. When you find the right loop, just drag it to the track panel and it will align there or even make itself a new track if you drop it outside existing tracks. Pretty nifty. Personally I prefer VSTi's like drum synths and samplers, but the process is still similar to above; now just using MIDI files (.mid). All you have to do is make sure the track containing the drum VSTi is selected and you can audition MIDI files in the Media Explorer, all in tempo with the project. You can find a lot of free MIDI drum loops around the net, but there's also reasonable at for example .