Will a RCA to RCA adapter hurt the sound?

I don’t think that anyone can say with any certainty, so the experiment suggested by R_F_Sayles would seem to be worthwhile. FWIW, though, my guess is that **if** a difference results it is more likely to be caused by the added length than by the adapter. Depending, that is, on the output impedance of the component driving the cable, the lengths that are involved, the susceptibility of the interconnected components to ground loop issues (ground loop issues can be sensitive to the resistance of the return conductor in an interconnect, and therefore to the length of the cable), and other variables.

I assume, btw, that you are referring to line-level analog interconnects.  Not to phono cables or digital interconnects, for which I would recommend against using an adapter.

Regards,
-- Al