Final answer:
The best time to use a sound signal when lost or injured is during dawn or dusk, due to lower background noise and better sound travel conditions.
Step-by-step explanation:
If lost or injured, the best time of the day to use a sound signal is during dawn or dusk. This is when the environment is usually quieter, and therefore, a sound signal can travel further and stand out more against the background noise. In contrast, during noon, sound waves can be more readily absorbed by the heat of the day, and during night, while it could also be quieter, there are often fewer people active to hear the signal. Meanwhile, morning hours can be effective, but are not necessarily the best when compared to dawn or dusk. It's important to remember that sound signals are a form of communication and the goal is to be heard by someone who can provide help.