You see the lightning before you hear the thunder because light travels faster than sound.
Literal
Thunder [subject-が] is-audible than first lightning [subject-が] is-visible-[nominalizer-の] [topic-は], light [subject-が] sound than fast travels-from [copula-である].
A textbook-style explanatory sentence with stacked structures. のは...からである is the cleft pattern 'the reason X is because Y.' ~よりも先に ('before X / earlier than X') uses the comparative より + 先に (earlier). 聞こえる/見える are the spontaneous-perception verbs ('be audible / visible'), distinct from 聞く/見る which require deliberate action. Worth a careful parse.