Hi Allen,
To play a subset of the Sounds in a Timeline:
1. Ctrl+Click on the Sound bars that should play (they will turn white)
2. Play the Timeline using Shift+Space bar (instead of space bar)
That should allow you to cache the Sounds. Another approach is to set the output of the unwanted tracks to No Audio Output; if they are not feeding into submixes, they will be optimized away the next time you play the Timeline.
To return to normal playback, ctrl+click in an empty space in the Timeline to deselect the white bars. Play using Ctrl+Space (rather than Shift+Space).
Please let us know how it goes. Thanks!