At a party, there are many people talking to each other. Your friend is standing at a distance and talking to you. You can hear him clearly. At the same time if you record all the voices reaching you, there is no specific voice of your friend on the tape. Why?

The reason for this discrepancy is your two ears. Diffraction of sound waves around your head as well as your two ears give you a sense of directionality of the sound waves. Also very unknowingly your brain is using the Doppler effect to pick voice signals from your friend - who is standing at a distance and talking to you. The direction information is lacking on the audiotape.

Hence at a party you can personally weed out "noise" from a "signal" but this facility is not there on the tape.

When a marching band is approaching you at a cross street, which sound do you hear first and why?

The long wavelength sounds of the bass drum will diffract around the corner more efficiently than the more directional, short wavelength sounds of the higher pitched instruments. Thus you will hear drums before you will hear a piccolo.