If you have added the music under Insert > movies and sounds > Sound from file:
go to the slide where you have added the sound,
click Slide Show > Custom animation,
Right click on the sound effect and select effect options.
Modify the stop playing after x slides from there.

If you have added under Slide transition:
Go to the slide where you want the sound to stop.
Click Slide show > Slide transition.
Under Modify transition, click on the sound drop down bar.
Select [Stop Previous Sound].

See if it helps.