Question

the speakers are coherent and in phase

Answer 1

Path difference $\Delta x=\frac{\lambda }{2}$
So, phase difference $\Phi =\frac{2\pi }{\lambda }\Delta x$
$\therefore$ $\varphi =\frac{2\pi }{\lambda }.\frac{\lambda }{2}=\pi$
Since the speakers are coherent and inphase initially, phase difference of $\pi$ implies that destructive interference takes place.
Resultant intensity $I_R=I_1+I_2+2\sqrt{I_1{I}_2}\cos \varphi$
Here, $I_1=I_2=I_o$
$\therefore$ $I_R=I_o+I_o+2\sqrt{I_o{I}_o}(-1)=0$
So, if the speakers are coherent and inphase, no sound is heard.