The quickest way to merge an audio file with a video clip while automatically removing the video's original audio is to use video editing software is to merge video.
A professional tools such as Final Cut Pro will allow you to import both your video clip and audio file, then easily synchronize them on the timeline. You can then mute or delete the original audio track from the video clip, leaving only the merged audio. This process ensures precise control over the audio and video elements resulting in a seamless combination of the two.
In more detail, start by importing your video clip and audio file into the editing software. Drag and drop them onto the timeline, making sure they are properly synchronized. Once synchronized, select the original audio track from the video clip and mute it or delete it, depending on the software you're using.