It sounded formal, like he was supposed to say it.

What comes across is a spoken version of a written communication; the words tend to sound stiff and formal.

He sounded more formal now, and Olivia regretted her words.

When you're speaking English, like you are now, you sound more formal.

I don't like the word mentor, because that sounds formal and rigid.

His voice sounded formal to the point of insult.

It would have sounded less formal in English but the meaning was there.

She sounded very formal, as if it were our first meeting.

Suddenly he sounded very formal, just as though he were addressing one of his patients.

In some cases, a postponed subject can be used to sound formal or archaic.