Combining two RCA audio signals

[LTig]

He means that the amplifier driving the sound bar speakers has floating outputs - this means that neither (+) or (-) speaker outputs are connected to ground. So you are probably shorting one side of the amplifier to ground, hence the hiss. The amplifier is probably a class 'D' which typically always have floating outputs (neither output is referenced to the power supply ground).
You will probably need to couple it with an audio isolating transformer.

He could try to connect the RCA grounds to the common ground of the soundbar, instead of to the - pole of the speakers.

 

[MasterHenaz]

Now I've tried a ground loop isolator (It was the only product that I could buy and try today)
Didn't work for me so I'm returning it but now I have to wait for a few days for the local electronics shop to order few line matching transformers for me to tryout.
This soldering I'm going to leave at more capable hands so updates coming next weekend.