Line Level is in the ballpark of 1V. Your +4dBu to +16dBu "pro levels" can be higher or lower depending on the volume.
In an actual pro studio environment you'd have meters and you'd know the dB level but in home environments we never know exactly. Most line output have more than enough output to drive most power amplifiers, and most power amplifiers have enough sensitivity to work with most line-outputs, etc.
The headphone output on a laptop doubles as a line-output so it's in the same ballpark, but typically on the lower-side if you're not listening at full-volume. Of course, it you have the volume all they way down, it's zero. 
You should get readable/measurable levels if you're hooked-up correctly. With the 5V default reference 1 count is about 5mV (0.005V) and you have ~5mV resolution.
P.S
I think you know this but remember you are sampling a waveform. i.e. If you run the Analog Read Serial Example code you're readings will "look random" and if you're not biased half of the readings will be zero. Of course that's why you have to find the peak, or calculate the average of the positive or absolute values, etc.
If you do run the Analog Read code, take-out the delay.