I'm afraid to say it's not at all possible. Since I'm sure your Sony won't have analog inputs to receive 5 channel information itself from another source. Also, you don't have digital inputs as well such as Coax/Optical Input
RCA stereo cables won't give you true 5.1 surround. You can get Dolby Pro Logic simulated surround, from them though. The only way to get true Dolby Digital and DTS 5.1/6.1 surround is with HDMI, Optical, or digital coax through digital connections.
Your sony DAV DZ120K has only RCA input and it will perform Pro logic simulated surround to get 5.1 effects, however just a quick question is TV HDMI (Arc) your input source here? Then u need device similar to one listed below, hope it helps.
I too have a similar question..!! If there is any standalone high quality DAC that can decode Dolby atoms atleast upto 7 channels... and can effectively simulate it onto stereo to headphones producing the best 3D effect for movies..!!
I have a Yamaha receiver which has the silent cinema through which I can output to headphones but I feel it’s just sending the side and back channels to the left and right of headphones..!!
So I wish to know if there’s any such DAC..!!