It seems that the low end of the pitch multiplier is clamped to around 0.4. So with both time dilation and pitch multiplier set to 0.4 things are in sync. At 0.35 the sound finishes sooner then expected. Also a 0.1 pitch multiplier sounds pretty much the same as 0.4.
Trouble is that I’d need a 4x-5x slowdown. :\