While you have a specific fragment playing in Song mode, you can “import” that fragment’s pattern configuration (e.g. with the right patterns selected, tracks muted/playing, etc.) by holding the value button in and long-pressing the track (01/Cd - 16/A8) you wish to switch to.
After switching to that track, the fragment configuration will keep playing (minus any scheduled play effects such as filters, fades, etc.). To switch to yet another track, switch to Live mode, then repeat; e.g. hold the value button in and long-pressing the track (01/Cd - 16/A8) you wish to switch to next. And so on, and so forth. This way you can work on a fragment and hear what it would sound like in Song mode.
Note that this also works for editing scenes in Live mode.