In Wave.video, you can record a voiceover right in the editor, when creating your video. In order to do it, simply click this sign on the timeline:
Just like with other audio files, you will be able to trim the voiceover piece, change its volume and add a fade in/fade out effect.
You can also upload a pre-recorded voiceover and add it to your video. In order to do it, head over to the step Audio >> Uploads and upload the file.