5.1 Surround Sound only outputting from one speaker on macbook

I have connected my old LG LH T360SE home cinema system to my white 13" macbook (late 2007) from the headphone jack via an audio Y cable into the RCA audio input on the back of the home cinema system. Unfortunately when playing movies via Plex, VLC or any other media player the sound only outputs through the front left speaker and sub-woofer. I checked that the other speakers are connected by perfoming a connection test in the product's setup and there is some background noise if you put your ear up against them. The 5 speakers work fine when playing a DVD on the integrated DVD player. Is there any settings on the macbook I need to change to make use of the full 5.1 surround sound?

Thanks