I definitely think it is more than just a myth. I don't think plants can "hear" as we think of it, but if we consider that everything around us is some form of energy (and energy wave) it would make sense that they are affected by different kinds of vibrations. A flower will turn to face the sun (the sun's light = an energy wave pattern) so it seems likely that sound could have the same effect.
Sound and light are the same electromagnetic field, vibrating in different frequencies. It's no surprising plants can detect sound.