In order to send sound to your ViewSonic ViewBoard, you need to know the board's device name.

Click on the triangle at the side of your board; this pops up a menu.

Click on the house icon.

You should be out here:

Look for, and then click on the vcast app.  That box is scrollable, like so:

When you click on the VCast app, that should get you to a screen where on the left side, it says Device Name. In the image below, the device name is Cast-1843:

Once you have found the Device Name, that's where you want your sound output to go. Click the two sliders on your menu bar:

Go down to sound. Apple hides the > arrow, but if you hover over it, an arrow in the right appears. Click the >

Under output, choose your device's name, such as Cast-1843, but I doubt that's your device's name. You need to use yours. In our office, ours is called vCastAudio

Once you have selected the proper device, you should hear sound.

If you used vCast Sender, you can send via that app, or if you used HDMI, then reseat the cable to return to your laptop's desktop.