Connect your older mac without an HDMI output to an HDMI display using this mini DisplayPort and USB male audio to HDMI adapter from Monoprice
This adapter converts the mini DisplayPort video output to HDMI. Stereo (2-channel) audio is obtained through the USB connection
Note that this adapter does not support more than 2-channel audio. Additionally, it is unidirectional and therefore cannot convert HDMI to mini DisplayPort signals. Finally, on some macs the video connector blocks the adjacent USB port