Changing fps from 60 to 30 before using multicam

You need to slow down the clip, isn't that right? I presume the audio that was recorded with the camera is speeded up so it sounds chipmunky, is that right? There's a couple of ways to do this, though I confess I've never done it.

Open each video clip using Clip>OpenClip (assuming 10.4 or later). Select both video and audio and press Cmd-R for the retiming bar. Change both video and audio to Slow>50%. The music should sound correct. Select the clips in the browser and try to build the multicam with the audio.

The other way is in the multicam clip itself. First, make sure what the actual frame rate of the video is. Is it 60fps or 59.94? Let's assume 59.94 for the moment. Build the multicam clip with just the video, without the music, which won't sync anyway. When you build the multicam clip use a custom setting that sets the frame rate to 29.97. The speeded up video with the fast music recording should sync up inside the 29.97 multicam. Select the clips in the multicam and use the retiming menu to change to automatic speed to slow everything down. Then add a lane in the multicam clip and sync the music to one of the slowed down video clips.