${y_1} = {10^{ - 6}}\sin [100\,t + (x/50) + 0.5]m$
${y_2} = {10^{ - 6}}\cos \,[100\,t + (x/50)]m$
where $ x$ is expressed in metres and $t$ is expressed in seconds, is approximately .... $ rad$
${y_2} = {10^{ - 6}}\sin \,\left[ {100\,t + \left( {\frac{x}{{50}}} \right) + \left( {\frac{\pi }{2}} \right)} \right]$
Phase difference $\phi$
$=[100 t + (x/50) + 1.57]-[100 t + (x/50)+ 0.5]$
$ = 1.07\,radians.$

Statement $-2$ : Due to the motion of source, wavelength of the sound waves (emitted by source) as received by stationary listener is affected.
Statement $-3$ : If receiver and source both are moving, the observed frequency must be different from the original frequency of source.
Treat motion of source or listener as always along a line joining them for all above cases.