Modplug can currently mix at max 96Khz in 32bit stereo. This is quite ok but if you use highfreq samples and play at very high pitch you can sometimes get a saturated signal and the sound gets funny.
This is easy to hear when mixing in 44.1Khz and you play hipitch samples.
I suggest that the you should be able to set a (insane high) internal mixing frequency and let MODPlug resample the internal mix to 44.1 / 96Khz or whatever before sending in to the soundcard. This way you can get rid of some saturation on highpitch sound.