Sound waves are energy that travel from one object to another. This gentle energy causes subtle vibration that stimulates a bean plant to germinate faster. A plant that germinates at an accelerated rate grows faster, and in some cases, taller.
No one has determined if plants prefer Mozart over Beethoven, simply that music stimulates their growth. Sound waves from music are gentle, therefore a positive sound for bean plants. As the plant germinates faster, roots become stronger and grow further into the ground. A bean plant with stronger roots has better chances of gathering water and essential nutrients.
Sound waves from speaking to a bean plant are not as rhythmic or consistent as music. It's not determined yet whether bean plants have a preference as to where sound waves come from, but most studies show that instrumental music has a positive effect on growth because it's gentle and continuous.