You can use Kdenlive to capture audio from a microphone while you play your project in the Project Monitor. In this way you can dub in a voiceover.
Under the Record Monitor, choose FFmpeg capture and enable Audio only and hit the Record button. Then move back to the Project Monitor and hit Play. You can now record audio only while the clip is playing. (This feature has had some issues in the past. It has worked in ver 0.9.4 - see bug #2910)