Hi Lomyn,
I was searching for another topic and noticed you hadn't received an answer on this.
I would suggest that you add a black clip to the end of your project and adjust its duration so that your full audio voiceover can then be covered by video. Drag the black clip from the Maps and Backgrounds panel (located in the bottom right hand corner). Adjust it's duration by clicking on the small gear icon at the start of the clip and selecting Clip Adjustments from the pop-up menu. In the Inspector that opens, adjust the duration to the required length.
Now you can drag the end of your audio to its end position. As you add your other video clips, the black clip will slide out beyond the end of the audio. When finished you can then delete the black clip.
In some sections you may need to split the black clip into separate parts, so that you can insert your video where appropriate. To split, right-click (Control-click) on the black clip and select "Split Clip". There are other methods to insert video (such as "Cutaway"), but I'll leave it there for now. Please post back if you need further information on this.
John
Message was edited by: John Cogdell - added last paragraph