After uploading or recording a voice-over, to delete it, your have two options: you can either delete in the voice-over window or in the voice-over editor.
Delete it in the voice-over window
Go to the Audio tab in the left menu, in the Voice-over window. In the Added section, click on the cross to the right of the voice-overs you want to delete.
Delete it in the voice-over editor
In the Audio tab in the left menu, in the Voice-over window, open the voice-over editor.
In the timeline, select on the recording you want to delete and click on the bin icon.
In the voice-over window, make sure you don't confuse the the recordings in the Added and in the Audio files section.
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If you delete a recording in the Added section, it's going to be removed from your video but will still be available in the Audio files section in case you can to use it again.
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If you delete a recording in the Audio files section, it's going to be removed from your project. It means that if you need to use it again, you'll have to upload it or record it again. If you previously added it to your video, it will remain in your video.