From the comments:
One LED's plus pin is connected to the left channel and the other LED's plus pin is connected to the right channel. The minus pins are connected to the other respective channels.
That means that they'll only light when there is a difference between the two channels. If you have a mono recording or a stereo recording with the same signal on both channels then you will have no voltage difference between L and R. I don't understand how you got this to work at all unless you have zero volume on one channel or you inverted the signal on one channel to maximise the difference in L and R.
simulate this circuit – Schematic created using CircuitLab
Figure 1. (a) What you've got. (b) What you need.
If you've got the same signal on L and R then you would have to wire as shown in Figure 1b but would need a higher output level than if you were running (a) with anti-phase signals.