moan, mown

Moan or moan means to make a low sound, from pain or despair--
They were startled when they heard someone moan.
He liked to moan or sigh at every disappointment, no matter how trivial.

Moan may mean the low sound, made in pain or despair--
He let out a moan of despair when he heard the news.
We can hear a moan, but we can't find the dog.

Mown or mown ( rhymes with moan ) is a past participle ( used with a helper verb, or to describe something  ) of "mow", meaning to cut grass or other plants to a low, even level--
He checked to make sure the lawn had been mown.
She liked the smell of new-mown hay.

Now that you know that, you can say--
"We heard him moan when he learned that his keys were lost in the new-mown grass."