I think I need a PCIe 8 bit sound card with a single audio out line

Do you know the HP model you have because 32bit PCIE slots aren't real thing. Neither is 8bit slots. Hard to help without knowing what you exactly need.

32Bit PCI slots are real, so is 8bit, but you looking at a 20 year old Intel Pentium 1 to find a 8bit PCI slot. I doubt you have that so a model number/type would help narrow down what your computer has in terms of slots.

PCIE slots are not called 8bit and 32bit. There are x1, x2, x4, x8, x16 PCIE slots.
You can put Asus Xonar DGX into your pc.

If you have 2.1 stereo speaker system, then you connect it to line out of the sound card. Nothing gets truncated.

Here's how to read speaker names.

The common naming system uses two digits separated by a period, like 2.1 or 5.1 The first number is the total number of speakers NOT including the sub-woofer. The second number is the number of sub-woofers.

Speaker system designs come as:
2.1 = Front left and right, plus sub
3.1 = Front left, right, and center, plus sub
4.1 = Front left & right, Rear left & right, plus sub
5.1 = Front left & right, Front center, Rear left & right, plus sub
7.1 = Front left & right, Front center, Rear left & right, Side left & right, plus sub

ALL of the output jacks are used for three connections. The male connector on the end of the cable going to the speaker has a Tip, Ring and Sleeve. Tip and Ring are the + contacts for two DIFFERENT speakers in a set, and the Sleeve is the common - contact. The way the jacks are always grouped is this:

One is Front left and right
One is Front center and Sub
One is Rear left and right
One is Side left and right

A sound card that can do UP TO 7.1 sound has all four of those jacks. My mobo's audio output is a 5.1 system, so the last jack (Side left & right) does not exist.

In the configuration tools that come with the audio card, you get to tell it what type of speaker system you are plugging into it, and it will send out only those signals on the correct contacts in the correct jacks.

Normally for a 2.1 speaker system there will be TWO cords running from the audio card jacks to the speaker system. Very often the big sub-woofer contains all the input cords, and then has output jacks that individual smaller speakers (e.g, Front Left and Front Right) plug into. One of the two cords from audio card to sub will be plugged into the audio card's Front (left & right) jack, and the other into the card's Center & Sub jack. Thus three signals can be sent from the card to the sub, and two of those will merely be forwarded to the separate Front speakers.

Now, you describe a "2.1 speaker" system that has only ONE cable to connect the audio output jacks to the sub-woofer in your speaker system, and then some cords to go from there to the two Front speakers. Are you SURE there is no provision for an additional cord from audio card to Sub? If you are, then this is a different version of the "2.1 speaker" system. The simple way to create something close to a true "2.1 speaker" system is to send ONLY the two Front (left and right) stereo signals to the sub box, and from the audio car's view that is a simple 2-speaker stereo system. Inside the sub box some electronics grabs the lowest-frequency signals from BOTH front channels, pulls them away from the "front" speakers, and re-routes them together to the sub-woofer. Although the signals sent from the audio card to the speaker system are NOT separated as a true 2.1 speaker system would do, the re-processing makes the speaker system perform a lot like the "real thing" and it sounds good.

So, IF you are sure there is only one signal cable to carry signals from audio card to your speaker system, it must be that last type. Then your audio card needs only to provide 2-channel stereo signal output, and the speaker system will do the conversion.