Playback sometimes doesn't play the sounds

This is what happens when you ask Audition to process everything at the same time - the more you ask it to do, the more it fails to do, especially with process-intensive effects. Fortunately there's a solution, and it's what that little lightning bolt at the top of the effects rack for your track is about. If you click on this, all the effects in your track will be pre-rendered before the mixdown, taking a lot of burden away from the mix engine. It doesn't mean that you can't change things if you need to - you can unrender the track, change whatever you need to, and then re-render the track again to get the performance back. If you do this for all the tracks with effects on, you'll notice a major improvement in playback and mixdown.