Try the latest revision, it should be faster than 4.0. Additionally, are you running this game at stock clocks (no overclocking?) If so, your CPU really hasn't kept up with the pace of Dolphin's developments. 2.4GHz from a C2Q 6600 simply isn't going to cut it for many games, even lightweight ones like NSMB Wii. You're most likely being bottlenecked by your CPU, hence the slowdowns and the choppy audio.
The latest development revisions have time-stretching that changes the audio according to the speed you're emulating the actual game, so it won't stutter or sound choppy. On top of that, the latest development revisions also run faster than 4.0 thanks to a handful of speedups. Just be careful of OpenGL on Nvidia hardware; its performance on higher IRs is spotty at the moment. You may have to go to D3D in that case; see which one is better for you.
Latest dev builds here: https://dolphin-emu.org/download
The latest development revisions have time-stretching that changes the audio according to the speed you're emulating the actual game, so it won't stutter or sound choppy. On top of that, the latest development revisions also run faster than 4.0 thanks to a handful of speedups. Just be careful of OpenGL on Nvidia hardware; its performance on higher IRs is spotty at the moment. You may have to go to D3D in that case; see which one is better for you.
Latest dev builds here: https://dolphin-emu.org/download