@Stephen > There is also a "Volume" control on the back of the DDJ-ERGO for the Mic Input. Could you please double check this level and increase it if necessary. Then see if it improves your Mic volume?
I'm not sure about a latency issue with the Mic Input. I've haven't heard of that... Are you sure you don't have the Mic input selected to a deck and possibly have an effect on it or something odd?
Have to see what Gavin or Pulse come up with on it since they have the DDJ-ERGO and can test it out.