Why isn't my Alert playing a sound?

There are three things to check:

1. Check the Alert sound setting. On iOS, open the task, tap the Alarm icon in the editor's bottom toolbar, tap Create New Alert..., and make sure Play Sound is on. On the Mac, edit the task's Alerts and make sure Sound Alert is enabled.
2. Notification settings are off. On iPhone, check iOS Settings > Notifications > 2Do, and make sure sounds are enabled.
3. Your phone is on Mute. Silent mode silences the Alert regardless of 2Do's settings.