Why doesn't the Bluetooth neckband have a common ground? Right side (R-) & left side(L-) is not common

I'm building a Bluetooth receiver from Bluetooth neckband circuit. i have to connect the output to 3.5 port so that I can plugin all wired earphones.

I have identified both Right side (R-) & Left side(L-) is uncommon. My multimeter didn't found any ground connection.
I merged Right side (R-) & Left side(L-) and connected to the speakers. It sounds ok but when I do right /left stereo check, it didn't perform good. while testing, there was a leak audio on the other side against the playing side.

please share your suggestion/solution.
 

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I don't know a whole lot about this. If you can find a signal ground in your device, you might use that with (+), and ignore (-). I think the technically correct way to do it is connect (+) and (-) to a center tapped transformer of the correct impedance for your device, and the center becomes ground. That may or may not be a good fit for your project based on size or expense. Search online for "convert balanced to unbalanced audio signal" for more info.