If your project uses an audio line-in input connected to your SlingStudio—but you have no video sources connected—then you won't hear any audio, and you won't see changes for Line-In Audio in the Audio Mixer VU meter.
This is by design. You must connect a video source to your SlingStudio unit, in addition to audio line input, before you can monitor audio line-in input or see changes in the audio VU meter.
This video source does not have to be chosen in either the Preview or the Program windows; it just needs to be present as a video source.
