The Intensity Pro outputs audio via HDMI (embedded) and analog RCA, simultaneously. You can use a 2 x female RCA to 3.5mm male audio jack to plug into your speakers or even the line-in on your Mac to get sound.
The 27″ Cinema Display only has DisplayPort right? I don’t think a passive HDMI to DisplayPort adapter would work on these displays, you’d have to get an HDMI to DisplayPort active scaler of some sort.