In a platform game, i want to know if there is anyway to make “3D Sounds”.
Spatialized sounds are possible through the “Sound Object”, look for it in the extensions… Of course they are 2D sounds 
Then you can place the object to determine the “sound source” position.
And how can i add a soundtrack, a sound without any position, to play as a background?
There are specific actions to play music, even in different channels where you can control index, volume and other things.
Exactly
To play background music, the best way is using .ogg files, and the action:
Actions ==> Audio ==> Play a music
or Actions ==> Audio ==> Musics on channels ==> Play a music on a channel
Analogously, there are actions to play sounds and play sounds on channels (.wav/.ogg files) 
Be sure to execute the action one time, otherwise you will hear the music/sound be played many, many… many times 