Are both sentences OK?
He/she was a singer before. But I never listened her/him before.

If I say,

I would love to have heard you sing.
I would love to have heard you singing.

Question

Are both sentences OK?
He/she was a singer before. But I never listened her/him before.

If I say,

I would love to have heard you sing.
I would love to have heard you singing.

Answer

No, a correct version would be,
I would love to hear you sing,
or
I would've loved to hear you sing.