His words must be listened to.

Explanation: When the subject of the sentence is the doer, the verb is in Active Voice. When the subject of the sentence is the receiver of the action, the verb is in Passive Voice. 

Here, the subject 'we' performs the action 'listening'.  If the given sentence is changed into the passive voice, then the subject in the active voice becomes the object in the passive voice. The word 'by' is introduced. Moreover, when the active form involves the use of an indefinite or vague pronoun or noun (somebody, they, people, we, etc.) as subject ; that is, when we do not know the agent or when it is not clear enough who the agent is, we do not use by + subject. To make the passive form of sentence containing modal, we use modal + be + 3rd form of the verb. So, the correct sentence is:

New subject (His words) + modal (must) + be + verb (listened).