“Why won’t my 1/4″ microphone plug work when I plug it into Mbox?”
When the Mbox inputs are set to “mic”, it only accepts signal from the XLR contacts of its input ports. In other words, microphones with 1/4″ TS or TRS connectors will not work. One workaround would be to try the “inst” or “line” settings instead, or to simply purchase a different cable with an XLR connection.