All 3 Uses

solemn
in
A Good Man is Hard to Find AND OTHER STORIES
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They stood close together, waiting; their faces were solemn but expectant and every eye was on him.†
Story 2. *

solemn = very serious--possibly dignified
Then he began "The Old Rugged Cross" and they listened politely but when he had finished they said, "Let us sing one!" and before he could start another, they began to sing with their convent-trained voices, "Tantum ergo Sacramentum Veneremur Cernui: Et antiquum documentum Novo cedat ritui:" The child watched the boys' solemn faces turn with perplexed frowning stares at each other as if they were uncertain whether they were being made fun of.†
Story 5.
She was better able to see the faces of the country people watching, the men more solemn than they were in church, and the women stern and polite, with painted-looking eyes, standing as if they were waiting for the first note of the piano to begin the hymn.†
Story 5.

Definitions:

(1) (solemn)
in a very serious (and often dignified) manner
(2) (meaning too rare to warrant focus)
Less commonly, solemn can mean that something was done with great or appropriate ceremony. It can also be used to describe something as dark or undecorated.