My corrections (highlighted in blue)
(a) A sale assistant showed us some pretty necklaces for example.
A sale assistant showed us some pretty necklaces.
(b) I heard a loud sound / a sound / some sound at my back. Correct
The robbers started to shoot in the air to make us frightened. The robbers started shooting in the air to frighten us.
(c) The burglars began to feel impatient. Correct
The burglars were impatient. Correct
(d) When I saw the burglars / robbers dashed into the jewellery,
they all were wearing the masks. I heard shooting sound at my back.
When I saw the burglars / robbers dashing into the jewellery,
they all were wearing masks. I heard a sound of shooting at my back.