Anyone have the detail on our mufflers?

I think the "variable volume" has to do with the dual exhaust. Under low demand conditions the exhaust gas is just expelled with one of the mufflers. Under high demand there's a valve that opens and the exhaust gas is expelled with both mufflers which reduces back pressure through increased flow.

If you look under the car you'll see that there's only a single exhaust pipe until it gets almost to the back end of the car where it splits into two pipes. Most conventional dual exhaust systems run two pipes all the way from the engine.

I found this on an old Nissan News website:

The variable capacity muffler system improves both engine horsepower at middle and high speeds by reducing exhaust pressure and reducing exhaust noise at low speeds. The system uses a special muffler flap valve – which uses high temperature aerospace spring technology and materials – that opens and closes according to exhaust pressure inside the muffler, switching the flow of exhaust gases from one tailpipe to two.

At low exhaust pressure, for example, while idling or coasting, the control valve remains closed. Exhaust is emitted without passing through a secondary bypass, through a single exhaust pipe. As the pressure becomes higher, the control valve (using aerospace industry spring and composite technology) opens, and the exhaust gas flows through a bypass – reducing exhaust resistance and exiting through the car’s dual exhaust pipes. The valve opens at 2,000 rpm and is always open at wide-open throttle.