How do you get audio onto a tv from a computer when the video is connected via HDMI - is it possible to get the audio using a jack to rca adapter?

That is exactly right. There is a jack that looks like head phones on one side and the other side splits into left and right audio plugs. On the back of your tv there should be audio plugs that are associated to the hdmi plug. Plug in your audio here. Alternately you can plug your audio dirrectly into your stereo. Or go from your stereo, into your tv or vice versa.