You need to insert -ing or to. Last night, I heard car brakes 1. screeching (screech) and people 2. (shout) in the

Here are the corrected sentences with explanations:

1. Last night, I heard car brakes screeching (screech) and people shouting in the street. (No change is needed here because «heard» is followed by a verb in its base form.)
2. When I looked out of the window, I saw a crowd of about twenty people standing around a young boy lying in the street. (We use «standing» and «lying» here to describe the actions of people, so the -ing form is appropriate.)
3. Next, I saw the driver of the car approach the crowd and kneel down by the boy; he was looking very anxious. (The -ing form is used to indicate an ongoing action, and «looking» describes the driver’s continuous state of being anxious.)
4. Watching the drama from my window, I began to consider the boy’s family. (Here, «watching» is used to express an action that was in progress while another action began, so the -ing form is suitable.)
5. Then a few minutes later, I saw a young woman running toward the scene and pushing her way through the crowd. (The -ing form «running» and «pushing» describe the actions of the woman as she moves towards the scene.)
6. Soon, I heard an ambulance siren screaming in the distance, getting closer and closer. (Both «screaming» and «getting» are used to describe ongoing actions.)
7. Then I saw the ambulance stop in front of my house. (No change is needed here because «saw» is followed by a verb in its base form.)
8. I watched the ambulance men get out and run to the injured boy. (Both «get» and «run» describe the actions of the ambulance men.)
9. Minutes later, I saw them run back to their ambulance with the boy on a stretcher, followed by his mother. (The -ing form «run» describes the action of the ambulance men.)

In these sentences, -ing forms are used to describe continuous actions, while the base form of verbs is used in cases where the actions are simple or momentary.