I've been using reaper for a few years and never had a problem. After a lot of time I decided to make the update of the program and, since then, it doesn't recognize MP3 files. I've installed Lame dll and nothing. I tried it on different folders (system32, Reaper, plugins). When I try to open a media file, MP3 is not an option. If I choose "All files" and select a MP3 it adds a small track with silence. When I try to render a project MP3 is not an option too! Audicity works fine! See images below.