not exactly sure, but double check the preferences under Edit Dropdown menu, then in the Audio I/O section make sure the proper recording & playback devices are selected, click OK (there may be other settings not set right too), check to see if that works...
I checked this using the 1.3.4 Beta that I hadn't used for a long time, it imported a couple of WAV files with no problems.
also tried Importing some MP3's in Flash 8 & it imported just fine...
could be you don't have your audio settings set up right on your system, but not positive on that.