Home cinema: Convert HDMI to HDMI + Audio
HDMI includes audio in it's signal.  Splitting off the audio isn't very hard, but recombining is, so most systems use one of two methods:


Some audio equipment has HDMI inputs and outputs.  These pass through the video to your projector and do the audio themselves.
Many (most?) projectors and TVs that have an HDMI input also have an audio output, either a headphone jack or optical output, that can be hooked up to your sound system.  This will pass the HDMI audio to the speakers.