When using certain dock-connector accessories, you may not hear audio from the line-out port. This article provides information for resolving this issue.
No audio from the audio line-out port
Some Dock Connector accessories may prevent audio from routing to the audio line out on the iPad dock. If this occurs, you can use either the audio out of the accessory, if available, or the headset jack on iPad for audio output.
Examples: Apple VGA Adapter, Apple Digital AV Adapter