I think these are a two fold requirement. Firstly to meet the noise reg's, a very quiet exhaust is required, which in sports cars, sort of defeats the purpose and secondly, to achieve the noise levels required, requires so e serious baffling to attenuate the noise level. So the bi-modal exhausts (obviously a marketing person came up with that, and engineer would have called it something logical like -switchable exhaust, or something in Latin), simply provides a diversion past the baffles - which increases sound/noise and most likely has the benefit of reducing back pressure (improving air flow), so you get slightly more power.
Some new cars come with "sound symphonisers" (another marketing wank word), which is a tube or funnel to "pipe" engine sounds up into the cabin, so as to increase the sound/noise level of the engine for the driver/occupants, because the car company has spent so much on NVH control that the car sounds dead and not exciting enough. I even think in one case, there is a microphone in the engine bay and a processor and speaker that then reproduces the engine tone in the cabin.
If you hanker for something different on a budget, you can buy plug-ins for your cigarette lighter socket that emulate the engine type of your choice while you drive. Fancy a V8 sounding 968, buy one and plug it in. Makes zero difference to performance, just engine sound inside the car. You can get V10 and V12 versions as well.
