your iPhone 3G is not notifying with a sound notification when new email arrived , Here are Some Tips and Fixes you may do to Solve This issue.

1.Make Sure volume on the unit is up and that the Ring/Silent switch on the left side of the unit was set to Ring.

2. check under Settings > Sounds to make sure New Mail was set to On.

3. If Above things has been checked and are okay, Then you may need to Perform a hard reset on the iPhone 3G.

4. After Hard reset, Your Problem should be solved