That isn't correct. Windows Vista only killed hardware acceleration for the ancient DirectSound API. rF2 is actually perfectly capable of doing hardware accelerated audio now, but since no hardware accelerator on the market supports any recent features we need, we chose not to use it. Also that makes it way easier to achieve a consistent experience for every PC.
rF2 actually uses quite a lot of cores these days. Just graphics is limited to single-core performance. However, sound, physics, AI, UI, all that sort of stuff each uses its own thread nowadays, so if you run out of cores, eventually that affects game performance regardless