I'm not quite sure that I'm envisioning your timeline correctly. Is each singer on a separate clip with its own embedded audio? Or is the audio detached? It might help if you could post a screen shot so we can see what you are trying to do.
Aside from that you can try Control-clicking on the audio clip and select Show Speed Editor from the drop down menu. That will reveal a speed adjustment slider handle in the upper right hand corner of the audio clip. Sliding to the left speeds up the clip; sliding to the right slows it down. Try making a slight adjustment to the speed of the audio clip by sliding the handle. Before doing that use the slider above and to the right of the timeline to fully expand out your timeline. You can do above procedure with the video clip as well, to slightly speed it up or slow it down to match the audio. The speed adjustments are so slight that you shouldn't notice any pitch changes in the audio.
Another thing you can try, is to convert your clips to H.264, Mp4/AAC with the free download, Handbrake. Do that even if the codec already is indicated to beMp4/AAC. Re-rendering often cures audio issues. Then import the converted clips into imovie.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.
-- Rich
