The baby is often heard .（哭）

The baby is often heard cry.
hear是系动词,后面可以跟do或doing,do表示经常听到,doing表示正听到.often是经常,所以用cry.

误人子弟，这么简单都错，笨蛋一个，错的，省略to的不定式在被动语态中要还原应是The baby is often heard to cry.

The baby is often heard crying.

to cry。。。。。。。。。。
被动语态。。。要将不定式还原

The baby is often heard to cry

ache heardache 宝宝经常头疼 哈哈