Built in speakers
If the installer did it right then there should be a pair of double jacks somewhere close. It's possible that they're located wherever the main entertainment center is, and the previous user would use an A/B switch to pick which room got sound. Unfortunately there are probably a half a dozen types of connectors. At least most will be red for one wire and black for the other.
I suggest pulling one of the speakers out to see what resistance rating it has, which is measured in Ohms or Ω. Common Ohms ratings are 4Ω and 8Ω. Some stereos will do both automatically, some will do both with a selector switch, and some will only do one or the other, with 8Ω being more common in residential applications. You'll want to set up your stereo appropriately or else if they can't be made to match, replace the drivers in the speakers or else the stereo.