What are the average voltage levels of microphone signals, LINE level
signals, headphone level signals, and speaker level signals?  What
precautions must be taken to input a Line level signal into a
microphone input jack?

If memory servers me correctly, it's not necessarily the voltages you
have to worry about. The key is impedance.

You don't state what you're trying to take signal from and shove it
into so that makes giving specific information rather difficult.

A quick Google search for "stereo headphone impedance" showed that
most models have a 32-ohm impedence, while
http://www.computerhope.com/help/csound.htm lists the microphone and
line-in impedances at 600-ohm and 47K-ohm (47,000 ohms).

To get different impedances to work together, you'll need a matching
transformer. For example, to make a CD player play through the
headphone jack to a microphone input without distortion and possible
damage to one or both of the components, you'll need an impedance
matching transformer with a 32-ohm to 600-ohm coils. It's been a
while, so I don't remember the winding formulae to make one yourself.

You might want to check your local Radio Shack or electronics surpluss
store to see if they have something that matches the bill.

Dell computer tech support tells me that, on my new laptop, I can use
the microphone input jack for a Line In signal into the Crystal S4250
sound card on the mother board. I did not want to "blow" the circuits
on the sound card if microphone voltages range in the millivolt range
and Line In signals are in the units volts range.  They mentioned
nothing about impedance matching (which I am somewhat familiar with in
going from 300 ohm TV twin lead to 75 ohm TV coax).  The other problem
is that I have discovered that the microphone socket is monaural and
what I want is stereo.  Dell wants me to buy a $200 docking station to
get stereo LINE IN when all I think I need is a PCMCIA audio
connection card which I have been unable to find so far (Dell cannot
help me here).  Thanks for your comment blitherakt-ga.