Creative T3100 speakers input mod
Stray inductive positive feedback and parasitic noise can be a difficult issue to diagnose when we are blind to your layout. 

You show two grounds.(left gnd right) and (V+ and Gnd)  

It seems the Op Amp (OA) has injected noise but from where?


Do some tests to find out where.


Short the input signal to local gnd with a sufficient  cap if it has DC bias
the connections at that noise may be insufficiently shielded.
Change ground for connections to the one between the stereo signal.
Keep wires short and twisted to reduce stray effects.

last resort, use an inverting buffer to make the stray feedback inverted