I believe you need an RF demodulator between the AC-3 out from the LD player and the input to the receiver. Three that are fairly reasonably priced, used, are sometimes available on ebay. They are:
Yamaha APD-1
Pioneer RFD-1
Kenwood DEM-999D
The Yamaha is probably the first choice. They sell for around $120-$140.
Yamaha APD-1
Pioneer RFD-1
Kenwood DEM-999D
The Yamaha is probably the first choice. They sell for around $120-$140.