These tracks complement each other. A stand-alone presentation is one that does not require a complement to convey its message. For instance, a radio program is a stand-alone auditory presentation. Based on these definitions, a radio program would not be consider a multimedia presentation, even if the radio program were accompanied by equivalents. Similarly, a radio program with an accompanying video track of signing hands would not be a multimedia presentation since the visual track is a functional equivalent of the audio.
Alternatives form a unit in a different way than multimedia components form a unit.
Calling the background audio a track seems less odd to me. Note that stand-alone audio presentations require alternatives so that they will be accessible to some users. Note that stand-alone video presentations require alternatives so that they will be accessible to some users. Note that audio tracks require alternatives so that they will be accessible to some users. Note that visual tracks require alternatives so that they will be accessible to some users.