Assign a stop action to the first and only keyframe.
Click the sound tab and highlight the loop you imported.
Click the "sync" tab on the left and select "event". The different settings here are defined in the manual.
In the "Loops" box type "1000". This will ensure that the lop will play through the duration of your visitors stay.
You'll notice I added a sound on/off button. This is a major forget me on a lot of sites now, but I think it should always be there. Just remember, not everyone has the same ears. This is a simple button that goes to a second frame in the movie. On this frame, I have assigned 2 actions. A "Stop all sounds" (to stop the music) and a "stop" command(to stop the movie). On this frame, the button's action's change to "go to & stop on frame one"(which starts the loop again. This is a very simple, but effective method for sound toggle.
Save the movie with a relative name like "loop" in the same folder.
Finally, export the movie by clicking file/export movie on the main menu. Save your movie and select the sound quality you wish to have. The higher the compression, the lower the sound quality and the smaller the file size.