HE ( ) in the classroom just now.he ( )be there now

hear sb do sth/doing sth 的被动语态为：be heard to do sth/ be heard doing sth.所以前半句选be heard to do.
前一句话的意思是刚才听到他在教室里唱歌.后一句的意思是：他现在可能在那（即教室里）.因为刚才还在教室里唱歌,但现在可能在教室里,但也可能已经出去了.所以后半句用may(可能)而不用must（一定）.