Hi Luke - For me, I just plugged in a dynamic microphone and it worked (a Shure SM-57). I know that isn't much help, but you have to use a dynamic microphone - not a condenser microphone as those require phantom power which the WK-7600 does NOT provide. Also, there is a gain button on the back beside the mic jack. Make sure it isn't turned all the way down by mistake. Get the mic working first, then move on to the recording. The second column of page 12 of the manual describes using an external microphone. (A Shure SM-58 probably would be better suited for vocals than the SM-57, but I'm not an audio recording expert nor do I play one on TV. 🙂
Enjoy your new keyboard. ~ Mark