There are a couple of considerations -
The Arduino input shouldn't be "floating". It should be pulled-up, pulled-down, or in the case of audio it should normally be biased at about 2.5V so the audio signal can swing positive and negative around the bias.
Audio signals are AC (they go positive and negative). The Arduino can't read the negative half of the waveform and it can actually be damaged by negative voltages.
There is a bias circuit at the bottom of [u]this post[/u].
With the bias and silence (or no connection) should read about 512. You can subtract-out the bias in software if you wish.
in my project the tip input is connected to the A0 input,
Don't forget to connect the grounds.
and read this signal input on the arduino.
You may know this already but since you are sampling a wave your readings will "look random", even with a constant tone. But, you will get bigger deviations from the bias with louder sounds.