How to amplify Stereo Headphone Outputs

[Bigglez]

dynamic mic to be used through the line input of the music computer.

Is this a professional mic (one with a three pin XLR connector?

While the condenser
mic worked well enough for the first few recordings

Are you sure that's really a condenser type?
Unless quite exotic its likely to be a crystal mic,
or more likely an Electret condenser microphone.
More here.

For live performances the mic is usually on a long
cable, and to work the signal is transformed to a
low impedance (150 to 600ohms) and made more
robust (against hum and pick up) with balanced
(two-wire-plus-ground) connections.

Some of these require a battery or phantom power
from the mixer board.

Studios can use more sensitive and delicate mics,
including condenser and ribbon types, which also
require DC power and are transformed to low-Z
for long cable runs. Some are 'tube mics' having
no solid-state amplifiers, and newer models output
AES/EBU digital data, no analog!

[CeaSaR]

Bigglez,

Working with standard consumer grade mics here, some newer, some
older, dynamics use either 1/4" or 1/8" mono phone plugs, no XLR's. The
condenser mic is the boom off of a computer headset (electret). All are
for use with the soundcard inputs on a repurposed Win98 machine, good
enough for his requirements at the moment. Professional gear is a few
years off. USB requirements need an XP computer, I forget what the rest
of the computer specs are. As with the pro gear, that is a few years off
also. Until then, analog is the way to go here.

CeaSaR