He took a step closer to her, and his voice dropped.

His voice dropped, like a stone into cold, still water.

He had looked around, being sure they were alone, and his voice dropped to match her own.

When there was no quick answer, the brother's voice dropped lower.

Her voice dropped so low I could barely hear it.

He glanced around quickly, and his voice dropped even further.

His voice dropped so low Michael could scarcely hear him as he went on.

Her voice lost its heat and dropped to a near whisper.

Now his voice dropped all the way to a whisper.

His voice dropped low enough that I leaned in to hear him better.