Here is some interesting information about a detailed experiment on the effects of music on plants.
Experiment Documentation
Interestingly enough, the experiment results seem to imply that the plants do not distinguish between pleasing music or jarring noise. Sound is sound and it interprets it at the soundwave level, unlike our sentient brains that attempt to rationalize music or noise. It is the intensity (volume) of the sound that atlers the result, and not the perceived quality of the music.
So in summary: My noisy exhaust fan and HAFs are music to my orchids' ears