Learn French: Il faut chuchoter, sinon on est exclu par le bibliothécaire. - We must whisper, otherwise we are expelled by the librarian.

Sinon is a single word that means 'otherwise' or 'if not'. It connects the two parts of the sentence to show the consequence of not whispering. You cannot write si non (two words) here; that is mostly used in short contrasts, like asking a question and saying 'If yes... if no...' (si oui... si non...).

Since the translation says 'we are expelled', why isn't there an 's' at the end of exclu?

Even though on is used casually to mean 'we', it always takes a third-person singular verb, which is why we use est. Because it is grammatically singular, the past participle exclu usually stays singular too. You might occasionally see native speakers add an 's' to make it exclus when they mean 'we', but keeping it singular is the strict grammatical rule.

How is the passive voice formed in on est exclu par?

It works almost exactly like English! You use the verb être (to be) conjugated to match the subject (on est), followed by the past participle of the action verb (exclu). The person performing the action (the librarian) is then introduced by par (by).

Could I say on doit chuchoter instead of il faut chuchoter?