No audio with mini-DVI to HDTV

Hi - I know this is a tired discussion but I haven't been able to find an answer. Can you help?

Here is my tv: http://www.lg.com/us/tvs/lg-42LE5400-led-lcd-tv

Here is a picture of the back of the tv:

I have a late 2007 (3,1) Macbook and an LG LED LCD model 42LE5400 tv. I'm trying to get audio and video to the tv. Video works perfectly but I have no audio.

I have a mini-DVI adapter connected to a HDMI cable running to the HDMI1 spot on the tv.

I also have a male-male audio cable with a y-RCA adapter (red and white ends).

I understand that the HDMI cable won't carry audio through the mini-DVI and that I may need external speakers but I want to explore all the options before resorting to that, since I don't have a stereo sound system. I do have some cheapie desktop computer speakers, however.

So. Which jack to I plug the audio cable into? I've tried lots of things, including the regular headphone jack. Still nothing.

Thanks for any help!

Sara