Usage patterns
〈独り言〉を言うto talk to oneself
He is always talking to himself.
〈独り言〉をつぶやくto mutter something to oneself
She muttered a quiet remark to herself.
〈独り言〉が多いto talk to oneself often
My grandfather was a person who often talked to himself.
〈独り言〉のようにas if talking to oneself, in a murmuring way
He murmured as if talking to himself.