I am making a single player map, but I don't know how to add songs for an especific area, in multiplayer is easier, because I make a bsp for every song, but as the maps are long, I will add checkpoints. but when I set a song, the song is heard in all the bsp.

How can I add different songs per area in the same bsp? Must I use trigger? (I don't remember its name)

You can set the background music for each individual cluster. You'd have to separate the BSP using portals or separate BSP tags.

You can also use (sound_impulse_start) and (sound_impulse_stop) to force music through scripting.