Question

when pendulum is vibrating then no sound is heard why ?

Answer 1

Human hearing range is from 20 to 20000 Hz, although there is variation between individuals.

Let's consider for example pendulum with the 5 cm length. The frequency of oscillations of such pendulum is

$$F=\frac{1}{2 \pi \sqrt{\frac{l}{g}}}= \frac{1}{2 \pi \sqrt{\frac{0.05 m}{9.8 m/s^2}}} \approx 2.2 Hz$$

So we can see that the frequency is too low to we can hear the sound.