which A/V sound is better?

the pioneer DVD I just bought at the thrift store has optical digital (little rectangle) and a coaxial?(RCA) audio connectors on the back

which is better for 5.1 or 7.1 sound?

the dish network HD boxes also have an optical digital connector.

do they make home theatre receivers with more than one optical digital input?

thanks

COAX and OPTICAL are the same type of sound connection DIGITAL / COMPRESSED

Theoretically there is no difference between them as far as sound quality. There are other differences like cost. Fiber optic cables are far more expensive than RCA cable (capable of delivering digital signals) but COAX/RCA cables are copper and capable of signal interference and loss thought not likely in a normal setup.

I prefer Optical simply because I know that its the only thing connected to the DVD player but still I get 7.1 channels of sound through a single red light.. fascinating. Either coax or optical is fine for digital audio.

Yes, most AVRs should have multiple digital audio inputs. it's better to use an optical digital cable.

it's sad to say tht most receiver on the market only accept one optical digital input. so you have to connect your other device via the DigitalCoaxial or normal RCA(analogue). In general you're not going to notice a differance either way. If you want to get really nit-pickey with it (not always a bad thing :) ) i have read that for short distances optical is worse because there is processing on each end to convert it too and from the light signal, but for long distances it is better becase there is no interferance and much less signal loss. I would say unless you have super human ears, go with whichever you have an open connection for on your receiver.