BMXTRIX,
That sounds simple. I'm not seeing how this relates to the speaker wires. Am I missing something? Any risks involved?
What are you using as your source? If you are using the internal FM tuner, then this won't work, and you will want to ensure that the receiver is set to output 'MONO' audio.
If you are using an external source, like a CD or MP3 player, or something else, then this will convert the 2-RCA outputs of that source to a single RCA input you can run to the same channel your speakers are using.
This clearly was a very amateur hour installation by someone who doesn't know commercial audio. It's also extremely unlikely that they ran the proper speaker cabling rated for use in ceilings, or used speakers that are of the proper type for what may be a plenum space. All of this is only a big deal if there is a fire.
If you do hear audio, and it seems to work fine, then go to the Onkyo website and download the manual. I'm having a hard time finding a TX 8060 from them. The 8050 I find without issue, but not an 8060.
Looking it seems the 8050 is a stereo receiver and the manual for it is here:
http://www.intl.onkyo.com/downloads/manuals/pdf/tx-8050_manual_e.pdf
Page 20 goes over the different sound formats. With the remote, you can toggle through different sound options such as stereo and mono and direct. You really want to use 'MONO', and it may be worth buying a remote, or contact the previous tenants to find out where they hid the remote control.
It seems like some sort of impedance matching volume control must have been used which corrects for the number of connected speakers. Not that this is good or proper, but it sounds like it works. All you really need to do now is get the receiver into mono mode and Onkyo, in their wisdom, apparently only allows this feature through their handheld remote.
Which (for the 8050) is right here:
https://www.remotes.com/onkyo-p/ONKYO-24140816.htm?gclid=Cj0KCQiAtJbTBRDkARIsAIA0a5PPDKMnGvGRxfbCeXgU1GKazxwfjl7uM7sS3T08LjSVh7JEn-smdisaAkggEALw_wcB
If it is actually a 8160, then you will want to get the remote for that, but it seems to function similarly.