Put the MP3's into a Blank Movie clip, then when you need them just call on the movie clip. I dont deal with sounds, I dont like sounds, so I tend to avoid sounds.
You can also do it with actionscript. Select the sounds in the library right click on each of them and select linkage. Select export this symbol and give them names "boo" and "app".
you can then call the sounds from your script by
sfx = new Sound();
sfx.attachSound("app");
sfx.start(0,1); // to look the sound once use sfx.start(0,20) to loop 20 times etc.
and for the boo sound effect
sfx = new Sound();
sfx.attachSound("boo");
sfx.start(0,1);