What sound device is in this line?

"They ripple tensely, they can hardly contain their happiness"

The sound device on that line is called anaphor.

Explanation:

Anaphor is a figure of speech that is added to a text as a sound device. The anaphor is perceived when the same word is repeated on a line, thus creating a harmonious sound on the line.

In the line shown above, we can see that the word "they" is repeated, creating a sound in the line.