Hey @Vizca75, have you already tried to contact Stadia support regarding your problem with the controller? They can help you with steps to try and also direct you to the Google Store team in case that the controller is defective and needs to be replaced.
You mean, when you plug in a 3.5mm headset the audio on the Chromecast doesn't stop and switch to your headset? Does the audio continue on the Chromecast screen after a headset is connected or does it stop and then not come out of the controller headset port?