I was creating a little song for one event, and when I started to add DPCM I heard that it's... a little too quiet. And that's the problem.
Normal wav file I imported was pretty loud.
What can I do with that fact?
This seems rather normal, DPCM coding works best for low frequency content. Higher frequencies tends to get attenuated and especially when lowering the quality setting.
There is not much that can be done, except lowering the overall volume or choose new voice samples.