I'm guessing that these are mp3 audio files. They may have been sampled at a rate that Flash doesn't like or it may mean that the file format is one that Flash doesn't like. I've seen this a lot and my solution is to open the offending file in a sound editor, check the sampling rate, change it, if necessary, and then resave the file. It will usually import after that exercise.
You simply have to resave the mp3 file by altering the sampling and bit rates. Depending on the software you are using for resampling the problematic audio files, you will have different sampling rates such as 44100, 48000, 32000, 22050, 11025, 8000 and bit-rate variants such as 128, 64, 32, and so on up to 448. The more you compress a sound and the lower the sampling rate, the smaller the size and the lower the quality. And, I would suggest that you should experiment a little to find the optimal balance between sound quality and file size. See this Help File for more about Using and Exporting Sounds in Flash Professional CS6. If you are using Adobe SoundBooth, then a peek in to this forum thread might help you alter Sampling Rates of your MP3 files.
Ok, I figured out what was wrong. It was resampling the file to 32kHz.
These settings work:
lame -q 0 -V 8 -b 32 -B 320 --resample 44.1 -m j sample.wav sample.mp3
This produces a file that not only works with Flash's import, but is very small in size (28kb VBR vs 118kb CBR) and retains all high-end crisp.