Like our Trunk-Slamming friend said, replacing with a newer A-Bus system would be the easiest. Then add a SONOS (or whatever streaming device) as a source for the updated user experience.

Since whoever wired the home, didn't have the foresight to properly put speaker wire in with the cat5, not much else you can do. Doubling up the pairs on the cat5 to get 18-20awg speaker wire out of it, still isn't going to be a high powered system anyway.

Depending on the decor of the home, Fins's suggestion of adding SONOS PLAY3s or 5s to the house instead, might be a good option. Although not much money for you in that and not much in the way of future upgrades either. As they will market directly to the customer with special deals and he can add things himself if he wants.