If they were beams of light, then you'd see both beams pass you at
the "speed of light", no matter what your own speed happened to be,
and no matter what the speed of the sources happened to be.
But since they're beams of sound, The sound from the source behind you
will pass you at 0.5 times the "speed of sound", and the one from the source
in front of you will pass you at 1.5 times the "speed of sound".