Oscar,
Insert - movies and sounds - sound from file - browse to your folder -
select your music peace - hit OK. Choose wether you want the piece to play
automatically or when clicked.
If you want the piece to play over several slides, activate the custom
animation task pane, double click your music animation. On the first tab
choose for the file to stop playing after XX slides instead of "After
current slide" or "on click".
If you want the little icon of the loudspeaker to be invisible drag it off
the slide into the gray area.
Luc