I've not checked with the PS3 to see if it's possible to keep the audio on the old AV cable but the Video on HDMI, as i've only ever tried it with the audio going via HDMI to my TV it then goes into the amp via the audio outputs on the TV.
If no one else offers beforehand I may be able to look at the settings if I remember (can't do it now as people are watching the TV)
With regards to the SPDIF this isn't as far as im aware optical but rather a different kind of digital audio connection.
I looked on the back of my paperweight (I no longer have BT broadband so that's all the vision box is at the moment) and there is only a SPIFDIF not an optical.
If you have an AMP/AV system that can handle either an Optical or a digital Coaxual input you could connect it to the PS3 (I believe it's possible to get a box of electronics that can take a digital optical signal and change it into a digital coaxual signal, think they cost bout £20 or less from somewhere like maplin)
You might even find there is a setting on the TV that controls which input it looks for the sound on when your on the DVI/HDMI feed.
also does the TV support HDCP becuase if it doesn't you will probably find as soon as you try to play a movie it wont let you as the tv isn't support the encryption for the copy protection
