In this case, we need to know if you are using Active PC speakers meaning they have their own power or are they passive meaning they have no extra wires other than just the speaker wires.
If they are active, something like the Logitech Z906 which needs its own power from the wall, then yes, RCA, 3.5mm, it is all fine. It is just a medium to transfer the binary bits across the cable which is converted into analog signal in 'steps'. Then once it reaches your powered PC speakers, it is then reconverted to 1s and 0s. So there should theoretically be no signal degradation at all.
As for passive speakers, of cos the RCA method will not work. You need an amplifier to give the speakers enough juice to play the audio.
That is why we need more info on what kind of speakers that you are using in other to give the best routing of audio from the set-top box to your PC speakers.
PS. The reason why I opt that you go the RCA route is because there's not much gain in using a HDMI extractor when you are just only going to play the audio in Stereo. So RCA or even optical SPDIF cable will be sufficient for your usage.