This extension involves the detection of changes in sound intensity. Your ears can detect nodes and antinodes, but or this investigation you may want to build a Sound Level Meter to get intensity measurements.
You must choose the sound frequency with care. If the wavelength is too short (high frequency), the nodes will be too close together to distinguish. If the wavelength is too long (low frequency), the distance between nodes will be greater than the size of the space. Remember that the distance between nodes or antinodes equals one-half of the wavelength of the sound, and the wave equation relates wavelength to frequency.
In extension #2 & #3 the length of the cardboard tube was adjusted to create standing waves. You will probably not be able to adjust the size of a room or hallway. Therefore, you must adjust the frequency to the room. The speaker is an antinode. At the opposite wall, you want a node.
The tone that you amplify must be as "pure" as possible. Viewed on your oscilloscope, it should look like a sine wave. Most tones contain harmonics. In other words, they are a combination of many frequencies. Each frequency will create a different pattern of nodes and antinodes in the space. The combination of all of these will mask changes in loudness that you are trying to hear.
In this experiment, less is more. Four walls, a floor, and a ceiling is best. Furniture or other objects create reflections which mask the pattern that you are trying to create. In an ideal space, you can actually hear the nodes shift position when someone enters the room.
As the speaker volume increases, the difference between nodes and antinodes becomes easier to hear. However, remember to have mercy on your ears. They are very sensitive detectors that need to last a lifetime.
The sound from your 386 Amplifier and speaker will not be sufficient for this job. Most home stereo amplifiers and speakers are satisfactory. Your school probably has equipment used at dances and assemblies that will also be satisfactory.
You can try using your 386 Amplifier to create the signal that you amplify. However, this tone will contain many harmonics. Many science departments have signal generators or audio frequency generators. These will be much better. If you use a musical instrument, the flute is best. Remember that a tape recorder can record a tone which you can play back as needed.