What Dolphin version are you using? Mario Galaxy should run fine on that rig.I get about 75% to 85% mainly. It's strage everyone kept going on about how Mario Galaxy with Dolphin works great, so I go and get a 4670k i5 which is overclocked to 4.4GHz, then now I find out Mario Galaxy works fullspeed with no music or 80% 75%-85% with music. I wish people were more clear, oh well hopefully other Wii games work good.
In order for the audio to play correctly, the game must run at a 100% speed, otherwise you'll get crackles, etc.