But the original PS3 can still internally decode and send the signal out as LPCM over your HDMI connection. You'll get the full HD audio but your receiver will say something like "Multichannel Audio" or "PCM" instead of "Dolby TrueHD" or "DTS -HD MA" Look here for details on setup and configuration:

http://hd.engadget.com/2009/04/21/hd-101-how-to-use-dolby-truehd-and-dts-hd-with-your-ps3/

How many channels you have hooked up to your receiver really doesn't effect your ability to listen to HD audio. Most HD audio is still 5.1 (there just aren't; that many 7.1 setups to justify much above that). Only about 10% of Blu-rays contain 7.1 channel versions of Dolby TrueHD or DTS-HD MA. 5.1 systems just down-mix this information.

In order to get 6.1 or 7.1 audio from most Blu-ray audio tracks you'll need to enable some form of matrixed sound decoding on top of the HD audio (Pro Logic IIx, Pro Logic IIz, or DTS-ES or DTS Neo:6). Keep in mind that the channel information you'll hear above and beyond 5.1 is being constructed by a processing chip, not a human being. The results can be good but can also turn into a jumbled mess.