No Sound from Inputs - Can't Isolate Problem

This could be a Sonar X1, V-Studio, or external equipment problem.  I may be overlooking something obvious, so please help me.
 
1.  A synth plugged into the #1 and #2 (1/4") inputs of the V-Studio interface suddenly stopped playing. 
2.  The MIDI monitor on the synth flashes to show it is receiving input.
3.  I can hear the sound from the headphone jack on the synth, so the synth is playing. 
4.  There are no lights indicating incoming signal on the interface.
5.  Although I did not expect two chords to fail at the same time, I changed cords.  That did not help.
6.  I next plugged a different synth into the same inputs on the V-Studio and I could hear it, which eliminated the possibility of bad inputs.
7.  I took a splitter cable (stereo 1/4" plug on one end and two mono 1/4" plugs on the other) and routed the input from the headphone jack to the inputs on the V-Studio.
8.  Now here is the weird part: this makes the lights flash on the interface, but I still hear nothing and there is no signal on the meter of the audio track in Sonar. (For the record, I tried everything from fully padded to wide open on the mic pres.)
 
That last point really confuses me.  I manage to get a very strong signal showing up on the intercace LEDs, but still no sound coming into Sonar.
 
What am I missing? 

Thanks, FBB. The unit in question is a JV-1080 4x and I did go into the menus and try rerouting to the other outputs. (The JV has three sets of stereo outputs). The very strangest thing is seeing the lights on the interface without hearing a sound when using the headphone patch. I may have a little amp around here I can use to try your suggestion.

I am about 95% sure that this was resolved by Sweetwater via remote connection and the problem and solution are interesting.
 
As you know, the V-Studio 700 interface has a built-in Fantom synth.  The Fantom has accepts ARX cards (expansion cards) which I do not have.
 
Somehow, ARX got selected as an available device and was off-setting my audio routing; that is, even though a synth was plugged into inputs 1 and 2, and the input 1 and 2 segment LEDs were flashing, no audio was coming through the V-Studio gear.  Once the ARX option was deselected, things were OK, although with multiple erratic issues going on, this was not a straightforward analysis.  Kudos as always to Sweetwater.
 
Remaining mysteries: 1-why and how this got selected and 2-why one of my older synths (JV-1080) still only plays back one MIDI channel at a time (even in 16-track Performance mode).  Current theory is that brief power glitch during a storm may have caused a reset in the JV, which will take me a while to fix since I have forgotten how to use the godawful interface. :-)  Possibly the ARX issue was created when trying to fix Preferences in response to the JV issue...
 

UPDATE: ISSUE IS NOT RESOLVED. I started Sonar today and the problem is back even though the Preferences are theoretically correct. I even restored the Preferences file from a back-up Sweetwater made. Apparently, this was not resolved and was only working temporarily because the problem is intermittent. I should have suspected this since some projects worked and some didn't and the behavior was not consistent. If Preferences were broken, they would always be broken. RECAP: MIDI goes to synth. Segment lights flash on V-Studio IO unit indicating sungal coming in. No sound is metered on the audio return track and heard from the console. Headphones in synth verifies synth is playing (and the same synth is not always the problem anyway). NOTE: The two green lights called MAIN below the segment LEDs on the IO are not lit when the problem is happening. UPDATE: MIC INPUT IS ALSO DEAD. SEGMENT LIGHTS SHOW SIGNAL TO IO BUT NOTHING COMES BACK TO AUDIO TRACKS.

Have you swapped any USB ports around recently? Although usually it doesn't create huge problems, just a re-assignment dialogue loading up projects.