You are using Win7 64 bits and official drivers. If a sound is playing when ASIO is starting, even if you don't hear it, you will get this message. You must 1st stop the sound playing (even youtube or music or anything) before launching Reaper. Once Reaper is running you can play all music & video you want
(carefull, in Reaper deactivate the "release of the driver when idle" or something like that, or you will have the problem with every alt-tab)
You can also get the beta drivers
... But as AVID is selling a big part of MAUDIO, the beta boards are now empty. I have drivers ONLY for FastTrack USB MKII*, PM me your email if you need them.
*(6.1.9 are the last I got), they correct this problem and works quite well (had to uninstall/reinstall one time, very bad buzz in all audio)
and sorry for my bad english