You may have already thought of this but just in case you didn't.... does the "Y" wire that you are using, the 2 male connectors that plugs into the stereo jacks on the keyboard, combine into a STEREO female end? Most of those "Y" cables are MONO which would explain why you are only getting one channel to work.

What I use is a wire with a STEREO 1/8 inch connector at one end which plugs into the sound card, and it has 2 RCA plugs (the same connectors your stereo system uses) at the other end for left and right channels. I then use 2 "RCA to 1/4 inch" adapters that plug into the 2 outputs on the keyboard, real simple. The wire is actually something that came with a portable Sony Discman CD player and is meant to connect the Discman to a home stereo system. All the wires and adapters you need should only cost about $6 in total.

So.... are you sure those 1/8 plugs and wires that you are using are all STEREO??? You can tell if they are stereo by looking at the plug and seeing if there are either 2 or 3 contact areas on the plug. 2 = Mono, 3 = Stereo. And use the outputs of the keyboard, not the headphone one.

Again, sorry if I'm telling you something that you already know, but judging from your symptoms of only having one channel, it sounds like your cables are mono and not stereo.

D