99.9% of music should appear between or at the speaker. That's just physics. There could be room reflections causing the appearance of sounds outside the speakers but that would be unfortunate. As mentioned, using out of phase recordings can cause a sound to appear outside the speaker span.
Amazing that physics is not the same in 2 rooms....or perhaps physical acoustic is one thing and psycho-acoustic is another field you must consider to exist....
The timing of reflections is one of the acoustic factors with which the Ears/Brain translated sound impressions INTO standing objects located in space in the room...The distribution of the different pressure zones is another one...
The correlation between the direct TWO distinct wavefronts coming from each speaker for EACH TWO ears with a time delay and the back and lateral reflections frontwaves timing is what i used to create in my room the intimate headphone effect of a sound out of my head but incorporating me in the soundscape...(i use Helmhotz diffusers and resonators,ionizers,Schuman generators grid, and a double screen panels with many acoustic devices on it to reflect , absorb or diffuse sound in some balance....
Perhaps you must add psycho-acoustic to physical acoustic in your scientific reading agenda...
Sound waves are not ONLY "rays" coming apart from one another on or from the walls , they are also different zone pressures competing with each others.... The dynamic geography of these pressuures zones and the tiiming of reflections in this geography, determined what i listen to and interpret to be a musical instrument standing at my left or right, OUTSIDE of the speakers vertical plane location...In my room at least.... 😁😊
It is not a mere distortion, a trick coming from out of phase speakers that explain the soundscape appareance, which is NOT an illusion by the way , but a phenomenon emerging from the way the ears/brain read and interpret the speakers/room relation through the recorded information transformed in the waves of sound and ALSO the way the ears/brain read and interpret the recorded acoustic relation of the musical event play back through the room /speakers relation ....it is a double interactive acoustic event: the acoustic of the recorded album with the acoustic of your room .....
The best way for your room to disapear and let the musical playback be optimal is after being put UNDER acoustic control...If not, the recorded event will not be acoustically optimal and will stay a 2-D not differentiated object between the speakers and not a multiplicity of 3-D standing objects or musical instruments around you outside the speakers in most well recorded modern album....
The depth of the sound is not only described as imaging and soundstage in acoustic but also in the " listener envelopment " factor....
99.9% of music should appear between or at the speaker. That’s just physics.