I offer several facts for you to consider.
1. Most computer cards do not have speaker level/impedance outputs.
They will not have a power amplifier which is needed for speakers.
Perhaps enough for headphones, but then the jack would be marked for
them. I would bet large sums that the card you have has a "line
output". But read below.
2. Not all "line outputs" are created equal. In the professional and
broadcast world, where I happen to come from, line output means an
output that is capable of delivering between 0 and +10 dbm into a 600
Ohm load. 0 dbm (deci Bells, milliWatt) can be easily be calculated
as 0.775 V (V^2 / R = 0.001W). But that is into a 600 Ohm load, not a
high impedance. On the other hand, much consumer equipment, probably
including computer cards, will use the same Voltage level but with a
high source impedance. So, if it is attached to a 600 Ohm load, the
high source impedance will effectively be in series with that 600 Ohm
load resistance and the Voltage level will be divided down to a very
low value, perhaps around 0.05V or less. Hence a high impedance, line
level output can not properly drive a 600 Ohm load. Most consumer
equipment takes this into account and then uses high impedance
inputs, which limit this Voltage loss to perhaps a factor of 50% or
around 0.4V. Not ideal, but usable. The tradeoff is in noise pickup
in the lines between devices. The 600 Ohm impedance which was
standardized in the early days of AM radio with tube circuits, is
barely considered low by today's standards where a transistor output
can have an output impedance of 1 Ohm or less. But it still has a lot
more resistance to noise pickup than a 10,000 Ohm, high impedance circuit.
3. The trick in many modern audio systems is to use a very low
impedance output with high impedance inputs. If the cable lengths are
not too long, perhaps under 100 feet for most audio cables, this
works very well. And since the source impedance is low, the noise
immunity is still good.
4. Your card that is marked Left, Right, and Center Speakers is
probably some form of "line outputs" that are simply labeled for
those speakers, respectively. In all probability, they do not have a
speaker style signal and if you fed them to speakers you would have a
very low sound because the low impedance of the speakers would drop
the level way down.
5. Many, dare I say ALL, computer "speakers" are really are a
combination of a speaker and an audio power amplifier in the same
package. They are DESIGNEDfor "line level" inputs to match the "line
level" outputs of computer cards.
So, you probably have almost exactly what you need.
One more thought. In a professional environment I often used a low
impedance audio output to feed high impedance inputs. This has never
produced any problems and is almost the preferred way of doing it in
many circumstances. Audio signals do not normally have lines that are
long enough to produce the standing wave and reflection problems that
higher, radio frequency signals do so having the "proper" terminating
resistance at the receiving end of the line really is not necessary.
