Just bought/assembled a used nitro mesh kit. I've tried several different pairs of headphones and still get no sound from any drum/cymbal. All pads are triggering properly so I am assuming they're all connected correctly.
Phone plays backing music and is input on the auxiliary input. This backing music is then mixed with the drum sounds and output through the line-out jacks or the auxiliary out or the headphone out depending on the control module
You can also perform the same mixing operation at the drum amp or PA if they supports multiple simultaneous inputs. Studio monitors typically will not support this type of operation and mixing needs to be performed up stream.