Which program outputs only mono sound? Kega or in_vgm?
And which version do you use? (as precise as possible, please)
It works for me in VGMPlay, so it should work in the in_vgm WIP/beta.
A good file to test if the player supports stereo is Shui Hu - Feng Yun Zhuan, Stage 2. The DAC is panned to the right speaker 70% of the song.