OUT 3/4 of the Rubix 24 is only audio output from computer DAW software that supports multi-channel audio outputs (e.g. Pro Tools, Logic, Cakewalk, etc). Outputs 3/4 cannot be used to output incoming audio (such as INPUTS 1 or 2 on the front panel) and there is not a workaround to accomplish this. If using DAW software (audio software), refer to the software's manual for instructions regarding how to output to an audio interface's outputs 3/4 (such as the Rubix 24).
Note that there is not a way to output to 3/4 with software that just uses the operating system's sound settings (such as iTunes or listening to music/watching videos online like YouTube).