Is your sound as if there's mains hum superimposed?
If so a simple lowpass filter on the LDR may help. Artificial lamps have a lot of 50 ( or 60 - depends where you live) Hz amplitude modulation. Are the sensor values stable ? or to they vary a lot - this would be a hint... LDR's are slow enought to reject most of this, but perhaps not all.
of course this may be a coding problem - but we haven't seen your code.
Perhaps you could post it..
regards
Allan.