you will see the sound failures in the devices I have checked them so I start to check first my speakers that are not short-cut, or broken etc, then the cables, then my audio outputs, (voltages and signal) and something very simple but that It happens often is the input of the headphones is sometimes open, if you have the diagram it is easy to see that you get sound from the amplifier inputs if it reaches you is forward (speakers) but back (preamp) or more before more that voltages t I suggest you start looking for the audio signal since all modern electronics is by signals not so much by Czech voltages with an amlifier to track the lucky audio signal