Learn Spanish (Latin America): Muchos murciélagos volaron alrededor de mi casco.

The phrase alrededor de is used to say 'around' a specific object or place. You need the preposition de to connect it to the noun it surrounds (in this case, mi casco). If you were just saying 'they flew around' without mentioning the helmet, you would drop the de and just say volaron alrededor.

Could I use 'por' instead of 'alrededor de' here?

You could, but it would change the exact meaning. Por usually means 'around' in the sense of 'somewhere in the general vicinity of'. Because this sentence uses alrededor de, it paints a very specific spatial picture of the bats physically circling or surrounding the helmet itself.

Why is 'volaron' (preterite) used instead of 'volaban' (imperfect)?

Using the preterite volaron treats the bats' flight as a single, completed event that happened and ended. If the speaker had used the imperfect volaban (were flying), it would be setting the scene as background information, leaving us waiting for a main event to interrupt it (like 'Many bats were flying around my helmet when I suddenly slipped').

Why does the word 'murciélagos' have a written accent mark?

In Spanish, words ending in an 's' naturally stress the second-to-last syllable. Since the spoken stress in murciélagos falls on the third-to-last syllable (mur-cié-la-gos), a written accent mark is legally required to break the normal pronunciation rule.