I just got a custom cable wired from a five-pin XLR (AT825) to hook into my iRiver (stereo mini). But it has a loud hiss in it. Cable guy thinks it is an issue with ground wiring. Does anyone have any experience with wiring stereo devices (or AT825) into the iRiver that can shed light on this problem? Does the iRIver need a non-standard miniplug wiring to work properly? I know monoplugs need to be wired backwards somehow.

Any other clues or ideas?

educated guess here-
a lot of devices like the i-river supply a small amount of voltage at the mini stereo mic input, to power the little consumer mics that most people use with them. it's kind of like baby phantom power. sometimes this can cause noise with mics that don't use this "plug in power."

For mono you need to feed the ring and NOT the tip and ground the sleeve
Do you know if the cable you have used the shield for ground and left the returns open?

Jack, I am trying to get a STEREO mic working, not mono. It has 1,3,5 grounded to the shield right now, but that gave me hiss.

Check out this link which describes the mics need for transformer
http://emusician.com/mics/emusic_audiotechnica_atst/
check out the last paragraph