Learn Russian: Услышав громкий звук, студенты отвлеклись от лекции. - Having heard a loud sound, the students got distracted from the lecture.

In Russian, a verbal adverb like услышав can only be used if its logical action is performed by the exact same subject as the main clause. Here, студенты are the ones who got distracted AND the ones who heard the sound, so the rule is satisfied.

Why do we use the ending -в to make услышав instead of the typical -я ending?

The verb услышать is perfective. Perfective verbal adverbs almost always take the suffix -в or -вши rather than -а or -я. This suffix tells us the action was fully completed before the main action happened: they heard the sound, and then they got distracted.

Why doesn't the phrase громкий звук change its form here?

It actually does change form, but it is invisible! It is in the Accusative case because it is the direct object of the verbal adverb услышав. Since звук is a masculine inanimate noun, its Accusative form is identical to its Nominative form.

What dictates the case of от лекции?

The verb отвлечься (to get distracted) requires the preposition от followed by the Genitive case to indicate what someone is pulling their attention away from.

What is the root meaning behind the verb отвлеклись?