Yours, Etc.: Origins and Uses of 8 Sign-Offs
early 14c., "official who receives and examines accounts;" late 14c., "a listener," from Anglo-French auditour (Old French oieor "listener, court clerk," 13c.; Modern French auditeur), from Latin auditor "a hearer," from auditus, past participle of audire "to hear" (see audience). Meaning "receiver and examiner of accounts" is because this process formerly was done, and vouched for, orally.