| OK then, the stop action might be interfering.
Delete the layer with the sound file in.
Create a new symbol (Ctrl+F8) choose 'movie clip' give it a name and press ok.
Drag the soundfile from the library (Ctrl+L) into the new movie clip and specify 99 loops; sync mode: start ; effects:none in Properties
Click 'Scene 1' in the bit between the edit area and the timeline to go back to your main movie.
Drag the movie clip with the sound in it from the library into a new layer.
Export.
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