We usually investigate the SWF file you send us in this case. Most likely the problem is caused by the fact that your SWF file contains the sound file is in ADPCM format which is rather specific and is not commonly used. Current build of Flash Decompiler can’t export the Sound in ADPCM format from your SWF correctly.
However we are now working at this issue to enable our product to export Sounds in this format as well. Please, wait for the new build of Flash Decompiler which will support ADCPM sound extraction.
Besides, your SWF file might contain a sound in Nellymoser format which decompilation is not supported yet because of Nellymoser Asao codec peculiarities.