Hi @hanamichhi ,

Does the BT audio cut out when you insert the aux. cable? (the other end of the cable doesn't have to be connected to anything). If so I'm wondering if you might have a faulty aux. input socket or the wiring connected to it

The wiring circuit in a lot of BT speakers, headphones etc. is that the audio from the BT goes through contacts in the aux. input socket to the amp/speakers, which changeover the audio source from the BT to the aux socket when the plug is inserted.

To find out you'll have to open the headphones where the aux socket is and inspect it. The left earphone is connected to the 'tip' connector (and its' associated contacts of the aux socket) Just a thought.