Add audio to transition video footage online

How to add sound to transition Video Footage online ?

1.  Upload  the beginning of your video from your device   
2. Record this first part video   
3. Upload  the transition video footage you want to add sound   
4. Upload the audio file (mp3, OGG, WAV or FLAC format) you want to add to the transition video footage   
5. Record your video   
6. This will start the recording of the transition video footage together with the sound
7. When the video footage playing is ended your video will pause by itself   
8. Repeat the same process from step 1 to 7 for each other parts of your video
9. Download  your mixed video made with transition video footage with audio