When I attempt to import an audio file, GarageBand briefly flashes a dialogue box / progress bar that reads 'Importing File(s)'; the box disappears and nothing is imported. I do not receive an error message.
I have tried, in combination...
Converting files to other formats (including AIFF 44.100 kHz, 16-bit) both with iTunes and by burning the files to CD
Selecting the files for import from iTunes and from the Finder
I am able to play files in iTunes and even to preview them through the Media Browser in GargageBand. The files are not rights protected.