They were heard ------ in the next room every night.

B to talk
根据题意：他们每天晚上都被听到在谈论.
原句：We heard them talk in the next room every night （ heard sb.do sth)
被动语态：They were heard to talk in the next room every night
heard sb.doing sth 是指在某一个时间点发生的事情.