In this guide, let's assume the MP3 file is not existing separately for each slide, but rather as a complete audio file for the whole presentation. First, select all the slides to edit at once and choose the uploaded audio file. However, the portions to be read for each slide have not yet been set.
So, for the next step, click 'All' next to 'Narration > Playback Interval' to set the 'Falling Interval' for each slide. Once the work is done, you can automatically check that each slide's duration has changed. The trimming method is detailed in Playback Interval (Video Trim), so please refer to that for confirmation.