Here are some options not yet mentioned:
1. Get a 7.1 channel PCI or PCI-E sound card and use it for 4 rooms. Use the SPDIF output on your motherboard to an external DAC for the 5th room. Some PCI or PCI-E also have SPDIF output. Since SPDIF carries the clock info, using the SPDIF output on a PCI soundcard should keep the external DAC in sync. I have used 10 channels out of my Creative X-Fi Elite by utilizing the SPDIF output for subwoofer output to a Behringer DCX2496 for EQ. The subs always stayed in sync with the other channels. The SPDIF was simultaneous with the other channels so I don't think you can play different stuff on its zone.
2. Use a multi-channel USB DAC with SPDIF output. I have a Steinberg UR824 with 8 channels of analog output and 2 more via SPDIF. You also get 8 mic inputs for connecting a measuring microphone and 2 headphone outputs. You could use the headphone outputs for 4 more channels, too.
3. Get a Firewire card for your PC and buy an Echo Audio Audiofire12 for 12 channels of outputs. You can usually buy them from Musician's Friend for around $500 with the discount code.
3. Get a Lynx AES-16e card for the computer and output to two-channel or multi-channel DACs. The AES signal will keep all identical DACs in sync. I just bought the AES-16e, a two channel DAC, and an 8 channel DAC for 10 channels of output. One card will output 16 channels and you can install (if you have enough spots) up to 8 cards in a computer for 128 channels of output.
4. Buy a Focusrite RedNet card for the computer. It gives you 128 channels of output via ethernet. You then need to buy Focusrite RedNet DACS. This could get expensive.