onboard audio and digital speakers?

i recently purchased a intel d845pebt2 with onboard soundmax Cadenza audio. i have a set of boston acoustic 7500 speakers. they have four speakers and a sub. the speakers have one digital input via a 1/8-inch stereo jack. the on board audio has analog output, by means of three 1/8-inch stereo jacks. The other two outputs are digital outputs S/PDIF RCA connector and S/PDIF Toslink connectors. i tried the rca output with a rca to 1/8 stereo cable. the result was a fuzzy muted sound out of the front speakers. these speakers work perfect with the audigy digital out. i tried different software setting, none seem to change anything. can someone help?