you can't use a resistor to stop the led from being always on but rather use a capacitor.1st you must work out the resistance of the circuit while there is no sound output,then work out a capacitor size that will allow the led to pulse when there is sound output but not while there is no sound.Its called a "gain",they use them in the music industry for filtering out noise from songs,they use them for VU meter,not necessarily the same hardware or application,but it uses the same basic principle.
ryan0