Try using Chankast's AICA plug in for audio if you aren't already.If it doesn't help then change it back because it displays some vital information. Open nullDC.cfg and set 'Emulator.NoConsole=' to '1' to get rid of the console that opens when you run the emulator.The MMU version is not recommended unless it's used on interpreter mode with games that make use of the Dreamcast MMU and can be much slower. Try the non-MMU version if you haven't already. nullDC comes with two main executable files, MMU and a non-MMU version.
Download and install the VisualC++ 2005 Service Pack 1 Runtimes.Go to Microsoft's Windows update (in the start menu or control panel) and see if they're any video driver updates, sound driver updates, DirectX hotfixes, or anything else you see that has anything to do with video or sound.To get the best performance your system needs to be above the requirements, but if it's not nothing stops you from trying anyway. I have Windows XP, but according to this you should able to run nullDC too using Windows Vista.
