This could be a room "mode" which is sympathetic to the frequency of the vocals. In English, this means that the sound coming out of your speakers may start to echo, or resonate, causing the blurring effect.
Try walking around your room clapping you hands and listening for echos. If found, place some damping material on one of the opposing walls to kill the echo. Keep at it until all echos are eliminated.
Another cause could be improperly designed ICs or speaker wire. Cheap speaker wire (actually, the insulation or "dielectric") will accumulate electrical charge and at some point release it back into the conductor. This will distort the signal going to your speakers and you will hear it.
Obviously this is more likely to happen in loud passages like crescendos.
Try walking around your room clapping you hands and listening for echos. If found, place some damping material on one of the opposing walls to kill the echo. Keep at it until all echos are eliminated.
Another cause could be improperly designed ICs or speaker wire. Cheap speaker wire (actually, the insulation or "dielectric") will accumulate electrical charge and at some point release it back into the conductor. This will distort the signal going to your speakers and you will hear it.
Obviously this is more likely to happen in loud passages like crescendos.