First 6 seconds of audio get chopped up when mixing

I recently switched to using Audacity on Mac and when mixing a session of about 7 tracks, the render is complete but with the first 6 seconds of the mix being distorted and chopped up. There seems to be no base/stereo line and the sound becomes clipped. then after 6 seconds the rest of the track is perfect. What is this and how do I solve it?
thanks

I recently switched to using Audacity on Mac

From? We live on fine details. Which Mac, Which OS, Which Audacity (all three numbers). I’m typing this on a 15" MacBook Pro under OS-X 10.9.5.

Koz

Undo the Render. Press the |<< button to skip to the start of the project. If any tracks have empty space at the start, drag-select in each track in turn from time zero to the start of the audio then Generate > Silence… . Then try Mix and Render.


Gale