If you are still hearing an echo when you activate the zero-latency monitoring mode on the NT-USB Mini, you should mute your microphone channel output in your recording software.
Here’s an example. Let’s imagine you are adding the vocal track to a song. In your recording software, you have the other parts of the song - drums, guitar, bass and so on; you have created a track for the vocal and assigned the NT-USB Mini to this track to record. By default, your DAW will likely still be sending the output of the microphone back to the headphone along with the rest of the mix. You will hear this in addition as the zero-latency audio from the mic, meaning your voice might sound as if it has an echo or doubling effect on it.
To prevent this, mute the output of your vocal track in your software when you are recording the vocal. This means you will hear in your headphones two things - the full mix of the song, but without your voice, and the zero-latency monitor output of your voice. This makes for a more natural recording experience.
Once you’ve finished recording, don’t forget to unmute your vocal channel so you can listen to your song with the vocal track you just recorded in the mix. Also deactivate zero-latency monitoring so that you can edit your mix without any distracting sounds from the room being picked up by the mic and sent to your headphones.