It's not straightforward to convert between SPDIF and a stereo
pair of RCA plugs. SPDIF is a digital standard (SPDIF is an acronym
for Sony Philips Digital Interface) and a pair of RCA plugs is
analogue. It is possible to buy adaptors that convert from one to
the other; if you search for SPDIF to analogue (or analog)
convertor with google, you'll find a selection.