Although the meaning can sound pretty serious, many people use this term to be polite.

From the way you described it, the situation sounds serious.

He made it sound so serious for such a small request.

Look, buddy, if we hear anything we'll let you know, but it don't sound serious to me.

One of them spoke in a voice that sounded serious.

If you say he's out four weeks, that sounds serious.

Although the name sounds pretty serious, the condition typically isn't.

You should've heard them trying to sound serious after my funeral.

The company said those incidents sound more serious than they really are.

"He sounded serious about throwing us in the river to me."