1.主动语态中,hear只能接不带to的不定式或动名词作宾语即：
hear sb do sth:I heard her sing that song.我听见过她唱那首歌.
hear sb doing sth ：I heard her singing that song.我听见她在唱那首歌.
2.但是,在被动语态中,又要将不定式前加上to,就以上面两句为例：
She was heard to sing that song.她被人听到过唱那首歌.
She was heard singing that song.她正在唱那首歌时被人听到了.
所以be heard do 是不存在的.