The opening salvo of jangly guitar licks on “Ex Post Facto,” from Chris Campion’s new EP, is so arresting, practically spellbinding, it raises the question of the extent to which pure sound, at once propulsive, insistent, and melancholy, can be a character in a 4-minute- and-50-second rock ’n’ roll tale. Instrumentals can of course call to mind all manner of emotions, but what about embodying, say, futility, or striving, or loss? Any one of those could be standing over your shoulder as the disc spins.