"Please don't tell me I have to buy a new surround sound system to go with the new Blu-Ray player!"
You already know the answer.
Does the 'system' have a extra optical or coaxial-digital input? If so, you just run either a toslink cable or video cable from the BlueRay player to this input and you have 5.1 surround sound.
If the 'system' has a Left/Right RCA audio inputs (colored red and white), you MIGHT be able to hook up RCA cables from the BlueRay player to these inputs.
But - standard DVD's include ProLogic sound track which work with the Left/Right audio outputs. I dont know if BluRay disks offer a ProLogic track. Check the menu system of the player to see if it will down-convert the sound to ProLogic.