I googled the speed of sound and found that it only depends on the medium (just like the speed of light but with different parameters). I can't see how it doesn't pick up the speed of its source! I mean for the constancy of the speed of light the hole addition rule of velocities was modified to the relativistic one. So how to maintain the constancy of the speed of sound?

p.s. I'm so grateful to every single one answered this question cause she/he truly induced me to understand the subject.