@boju & kereberos you can not run a speaker from a line level RCA output and it was never a good Idea to try. Those are very low levels and need an amplifier to drive a speaker. All the adapter does is chenge drom digital to analog it will not give enough power for a speaker. Perhaps you were thinking the speaker had a built in amplifier, but that's not what I'm getting from this.
A cheap soundbar or even a pair of PC speakers would be the simplest option, just make sure it has an input that matches the TVs output. Or now that you have the adapter you could use anything with a stereo RCA input.