@max: hey! good to hear from you.
what kind of LEDs and power supply are you using? it may be that the strain on the supply is messing with the 555/audio. I used a separate power for the lights. They each pull 350mA so that's around an amp of rapidly fluctuating pull. that's enough to mess with the sound if they were on the same supply. You could also try just powering the LEDs off a separate regulator. or getting a supply with a greater current rating.