Even for a desktop computer microphone?
Almost all "computer microphones" are electret condenser types and they require 5V power (normally supplied by the soundcard).
[u]Here is an example[/u] of an electret microphone preamp circuit with power for the mic and biased output.
Is that like mixing 2.5V with my audio signal?
Yes. The Arduino can't read negative voltages, so and AC audio signal must be biased. (You can subtract-out the bias in software if you wish.) [u]Hre is an example[/u] of a bias circuit.