It sounds to me like you do have the interpretation of the interaction term down. You would include the interaction term if you think the effect of ConsDur depends on the level/value of Sonority. (If the interaction effect is positive) eg the effect of ConsDur is greater when Sonority=2 than when Sonority=1. The interaction term is basically a new variable that takes the mathematical value = ConsDur * Sonority.

You should include the main effect for Sonority too. People will wonder what changes in effect sizes would happen if you included it. I wouldn't leave it out just because AIC is better without it.