I used to use an ancient old Mac Mini (from the first release of the things) under the TV, using a DVI-to-HDMI connector to get the image up... and I found that if I ran the audio from the 3.5mm stereo socket on the Mac via a 3.5-to-phono cable and plugged this into the audio parts of the component input on the TV, then it would happily play the audio over the top of the HDMI signal. It's definitely worth experimenting around with any extra audio inputs you may have on your TV!
Later, I switched to using a newer Mini with an HDMI out, and it was actually far more problematic to get the audio working there! I struggled for about a week with the TV never showing up as an audio target in the Sound panel, trying everything the various Mac support fora suggested to no avail, until finally an OS update came along and it suddenly started working!