On mobile, you can turn sound effects on or off and pick which sound plays for each action. Open Profile → Settings → Sounds (or tap Sounds in the root Settings tab). Settings are per profile: each family member can choose their own sounds.
You can set a sound for when you add a task, when you complete a task, and when you complete all of today’s tasks (celebration). Choose from the list or set None to silence that action. Use the preview button next to each option to hear the sound before saving.
Example
If you find the default “task completed” sound too loud, open Sounds, tap the sound for “When you complete a task,” and pick a gentler option or None. Sound effects are only played on mobile (not in the browser).