No, you don’t have to upload it on each slide. Here’s how you can do a bulk upload ;
Step 1: Make sure you are inside your Interactive Deck Editor.
Step 2: Go to the tools tab.
Step 3: In the Voice-overs section, find an ‘Upload’ button. Via this button, you can upload all slides voice-overs at once. Keep in mind to name voice-overs as per slide numbers e.g. 1.mp3, 2.mp3, 3.mp3, 4.mp3 and so on.
Step 4: After the upload. Do, revisit the slides to confirm if the correct audio files have been uploaded.