I've heard old wives tales about this kind of thing before (mostly from my mother who is a) an old wife b) a gardener and c) a musician). I have a healthy skepticism, but I'm not totally ready to write it off. There have been a number of studies that show that sound waves have a positive affect on the acoustic properties of musical instruments. Conventional wisdom for a long time had been that high quality stringed instruments improve with age, and studies have shown that long term exposure to sound waves (or even short term exposure at high intensity) can change the cellular structure, and therefore the acoustic properties, of wood (I'm sorry I don't have citations at the moment and don't have time to look them up). It stands to reason that if sound waves can affect dead vegetable matter it can affect living plants as well. Besides which, sound is a physical wave and produces measurable changes in air pressure at sufficient volume. Consider also the use of ultrasound as a therapeutic measure in modern medicine, clearly people and plants are different, but it's a precedent. Presumably there is also a heightened electromagnetic field produced by the playback apparatus which can certainly affect living tissue, though not necessarily in a positive way (i.e. warnings about long term cellphone use, cancer clusters associated with high tension transmission lines, etc.). I see no reason why these can't all have a measurable effect on plant growth. There's also the "positive" vibe issue to account for on a more spiritual level. Am I saying that the results would be appreciable, no, but at least the "potential" exists for a perceptible difference. But I suspect it would be minimal, to say the least.