The speed of sound in air can be measured in many simple ways. Research how this might be done. You may find reference to cannon, hand clapping and microsecond timers among others.

Consider thunder and lightning. Lightning is a natural electric discharge which superheats the air and so gives a sonic boom because the superheating is so fast that the air is compressed: this sound is thunder. How do you estimate how far away the lightning is and so your safety? How does this work exactly?

Find out or measure the speed of sound in air and so calculate how far off lighning is for various examples.

This is a speed of sound calculator

Once familiar with measuring the speed of sound in air try carbon dioxide or other gases. See also this reference for a start on measuring in solids