Helen had to shout above the sound.

This sentence implies that there was a noise or a level of sound that was so high, Helen had to raise her voice in order for others to hear her. However, the sentence is incomplete as it currently stands; it might be improved by specifying what kind of sound she was trying to shout over. For example:

- Helen had to shout above the sound of the traffic.
- Helen had to shout above the sound of the music.

If you have a specific context or type of sound in mind, please let me know and I can help complete the sentence more accurately.