Final Cut Pro - The Hidden Value of Mixdown
When nesting, this is an essential audio step.
Final Cut allows up to eight levels of nesting, putting sequences inside other sequences.
However, when you nest, your audio sometimes gets confused, resulting in missing audio or bad audio levels.
Whenever you start creating nests with both audio and video, make a point to select Sequence > Render Only > Mixdown (or press Option+Command+R).
What this does is mix all the audio in all the sequences into a single stereo pair. This reduces the load on the processor and results in cleaner audio playback.
Note: Feel free to use mixdown when ever you have lots of audio tracks in a project, even if you don't nest. Mixdown will always result in smoother playback.