From the sounds of things, it could be one of a few things causing this issue, and I definitely have a few troubleshooting steps for you to try. Firstly, I'd make sure that you have the Red and White audio cables connected from your cable box to the Slingbox, and that the volume of your Macbook Air isn't turned down or muted. If you are using component cables for your video signal, you should have a total of five cable connections leaving your cable box (red, blue, and green for video -- red and white for audio).
Here's a helpful article on Slingbox cabling configuration:
Another step I would try would be to optimize your audio settings within your cable box. If, for example, you traditionally use an HDMI cable from your cable box to your TV, your cable box might not be sending audio out to the component outputs. The steps to get there might be slightly different based on the cable box you have, but under the settings menu, you should have some option to optimize the audio.
If all of the cabling is properly set up and the audio on the cable box has been optimized, the next step I would recommend would be to test the cables themselves. Try connecting your audio and video cables directly from your cable box to your TV. If the cables work without issue, then you can rule those out as the cause as well. You specifically mentioned experiencing the issue on your Macbook Air. Do you have another device available to try to connect with? That way, if it works on another device, we can isolate the issue to the Macbook. If that is the case, I'd try uninstalling and reinstalling the desktop app.
If all else fails, definitely give us a call. Our live support would be happy to assist you. Hope this information was helpful!