Little help, please & thank you

this is a project i have to make for school and i was wondering if someone could help me make a death system kinda thing. i already have it were i can kill the enemy but i dont quiet no how to make it were the enemy can kill me. i want it were if they hover over me then i die.
also i needed help with adding a backround sound that loops starting when the game is started but you can change from 3 different sounds using buttons the audio i wanted to use for those was sfx_sound1, sfx_sound2, sfx_sound3.

here are the sources:
http://www.megaupload.com/?d=Z3UF8XKV

any help will be greatly appreciated and plz dont just tell me what to do i wana learn

btw this is my first flash project.