You need to compress the audio files when importing them in/exporting the flash file.

When having a sound imported into your library double click the sound icon or right click/properties on the file to enter the properties menu. There you can set the compression mode for the file.

adpcm = best for button clicks and small sounds like that
MP3 = best for larger music files
Speech = best for speech

Just try different setting and press test to see what quality you are satisfied with.

Make sure when exporting the file that you dont check the override sound settings box otherwise the settings you chose for all the files will be disregarded.

But if you got a movie with just speech, then you might aswell DO choose override sound settings in the export menu and pick "speech", so that you wont have to configure all the individual sound files.