I got a microphone test that includes 99 audio files in .ape format, but Reaper can't import any of them. Play.app can play them and XLD (X Lossless Decoder) can convert them, so there shouldn't be anything wrong with the files.
Is .ape not supported on Reaper for Mac? It should be since it is listed as a supported format here:
http://reaper.fm/technical.php