Audio although nice is not as important (for most people) as video...
When I started working on computers the only sounds you heard were from the internal speaker of the computer. A low quality 2 to 3 inch speaker.
That is only part of the story.
The computer programs were incapable of producing anything beyond a few basic sounds of low quality.
It was about five years from the interdiction of the IBM PC and Apple II for someone to come up with a way to produce a add on device that could take the signal from a program, convert them from digital to analog and send the resulting signals to a speaker or set of speakers to produce the desired effect.
One of the reasons that the digital world didn't have a solution for sound right away is that the signal needs to be amplified to a point where you can hear it.
A digital signal that is generated by the processor doesn't have the amperage to move the magnet in a speaker for much more than a beep.
You could use the small speaker built in a computer for your sound but the quality of the sound would be very low, in addition some sounds would not be recognizable.
In the mid 1980's two companies introduced an add on card that would produce high quality sound, Creative Labs and Turtle Beach.
Unfortunately like all new technology it would take two or three years before they were affordable, by affordable I mean less than $200.
As the technology of micro chips advanced the amplitude and cost of these add on cards became cheaper and of higher quality, a side effect of these advances in technology is lower power consumption to a point where the motherboard manufactures were able to embed this feature on the motherboard instead of using an add on card.
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How ever if you are an Audiophile you will still want to use the add on card to drive those surround sound speakers because the embedded devices are limited in the amplification they can produce. The embedded devices do have high quality and moderate amplification for most applications but if you want to "Rock the house" and blow out yer ear drums then an add on card is required or an external amplification device.
The last add on card I bought was a Creative Labs PCI 512. This card had 512 Meg of ram to store data for processing.
Although the card could produce 20 Watts of power for four speakers (that is an 80 Watt amplitude) my speakers are rated at 120 Watts for treble and 200 Watts for bass. So I have an external amplifier for when I want to drown out the kids driving by with there boom boxes in their roller skate cars.
The quality of embedded devices has improved over the last ten or so years to the point where you do not need an add on card to produce high quality sound, and with amplified speakers the low wattage produced by the embedded device can produce high volume thus eliminating the need for the add on card.
My last motherboard upgrade has an embedded SoundMax Integrated Digital HD Audio driven by the Analog Devices 65525 chip set. This has more than enough quality and with the external amplifier system the volume for most of my needs. :)
As pointed out above the Audiophile would want to purchase the more advanced add on card to produce the high quality signal to drive those high quality speakers. :)
As always do your research before you buy, some of these devices cost thousands of dollars ...