I would be looking in the living room / family room where an entertainment system lived and probably supplied audio signals to those in-walls from a AVR with multi-room outputs or additional amps. The wires might go to a wall plate or come up through the floor. I would go into the crawl space below or the attic above those rooms as the wiring was probably added after the house was built to find them and follow them. I've done that with alarm wiring in my house.
If all that fails, there are in-wall cable locators with an injector and a tracker. They connect one unit to one end of a wire to inject a low power radio signal onto the wire and there is a hand held tracker you use to follow the wire - borrowed one from a tech at work years ago but I don't know if you can rent one or buy a cheap one that would do the job.
2-Ch (HT L/R): Oppo BDP-105 BD, Adcom GFP-750 pre, Bryston 10B Sub Xover, Bryston 4BSST2, Paradigm Signature S4 v.2, (2) SVS SB12-NSD subs, AQ & Cardas XLR
Home Theater: Bryston 4BSST2 amp / Paradigm CC-590 (C), Outlaw 7700 amp / (4) Def Tech UIW-RSSII (LS/RS/LB/RB), Samsung 46” 3D LCD