The gears in manuals are changing to make the engine move at a relatively decent speed while helping the car go faster. You can hear the pitch of the engine go up, and you will hear it keep going up until it is too high. That pitch is what you drive by because you will only change up to the next gear when you know that the sound is too high.
You change gear when the engine is revving too high, and it will shift back down to a much lower sound. You will hear the engine get higher and higher if you are still speeding up, and you will change to the next gear again. You will do this until you are at a cruising speed, or you will shift down when you are going too slowly to be in the gear that you are in. You can shift from one gear to the next to get back to first quickly, and you will hear the engine simmer down.
You can hear the engine slow down when you are coming to a stop, and you will simply go back to first gear if you are making an emergency stop. If you are parking, engage the parking brake to be safe.