I have not tried anything like that, but the approach should work, provided that the soundcard's internal mixer can route the input straight through to the output and provided that the levels are compatible.
Since the PC Speaker header is designed to drive a speaker, some attenuation might be required to prevent damage to the line-in.

Good point. I think PC speaker is driven by +5v which is hot. Since I'd be making my own cable for this, getting an attenuator in line would be no problem.

Also be careful of polarity. I haven't studied the PC speaker circuit on many motherboards, but at least in theory, you could end up shorting the +5V output to Gnd on the sound card. That would be bad.

A better solution would be to build a differential input, condition it through a DC-blocking cap, and give the sound card a 1V AC output referenced to Gnd. But that takes the project from "adapter cable" to "perf board" at the least.

Here is an article with some simple solutions to recording the PC speaker, trying to solve the 'hotness' of the PC speaker signal, and the polarity issue:
http://www.oldskool.org/guides/speakerrecording
It links to this: http://www.deinmeister.de/e_sbpcqlnk.htm
I think that may be what you want.

The circuit in the second link in Scali's post will work fine, but the 100ohm resistor should be 1k or perhaps more to get a level that's not gonna be super loud. Optionally a capacitor (1000pF...0.01µF)between signal and ground can be added too to make sample playback over PC speaker sound less noisy and perhaps make things sound closer to what an actual PC speaker would sound like.

i use this circuit, works well and i tested it in many other systems with different types of pc-speaker.
you may want to use a trimpot for volume also, and a capacitor on output.

Wow, what's with the voltage divider using 47k resistors? That's going to result in a pretty high source impedance for a line input.

Tiido wrote:

The circuit in the second link in Scali's post will work fine, but the 100ohm resistor should be 1k or perhaps more to get a level that's not gonna be super loud.

Why? As I understand this resistor does not affect sound attenuation, but only pulls up the signal to vcc, which can make the sound a bit louder.

The lower the resistor, the more current is produced (and chance of destruction increases too) and with that higher voltages are possible. 1kohm and higher will exclude possibility of magic smoke aswell. One PC speaker wire is +5V on most motherboards, you want to be careful with that 🤣.

Tiido wrote:

The lower the resistor, the more current is produced (and chance of destruction increases too) and with that higher voltages are possible. 1kohm and higher will exclude possibility of magic smoke aswell. One PC speaker wire is +5V on most motherboards, you want to be careful with that 🤣.

That is a totally different story about current, 100 ohms is too low for sure, but I was (you were) talking about loudness affected by resistor’s value. So I asked why you say it is affected, maybe I’m missing something. And yes, one pc speaker wire is +5v, so the resistor only pulls it up to +5v.

That resistor is what produces the output voltage according to current generated, it has direct effects around volume that is achieved. It essentially controls the gain of the output buffer on the motherboard. You can of course do low value resistor, mimicing the speaker and getting maximum voltage and then using voltage divider to get it down or drop that divider altogether and get right levels with just that one part (and this approach uses less power too though it doesn't matter in most cases 🤣).

if i make a circuit without the 47kOhm its waaay too loud.
i already tested this in several machines, and also creative uses this method on their vibra 16/sb clones.

With the diagram, this won't burn your card to a crisp as the PC Speaker header outputs high sound through the sound card and can make it sound muffled and way too loud.

Keep in mind that the values differ greatly, when it comes to the voltages that are spat out through the Speaker line.

Most desktops will have a voltage of about 2-3V going through the Speaker line whenever there are beeps happening. These values can be all over the place regarding of what machine we're talking about.

Example: I have a 486 POS cash register PC here that uses a 0.2W speaker. The reason being: The voltage coming through the Speaker line is a meager 0.04-0.10V whenever there are beeps. This is very low for a Speaker line, albeit the 0.2W speaker makes up for it by being so weak as well, resulting in this actually being a rather loud PC Speaker.

"Normal" PC Speaker values are between 2-3V and using 1-2Watt Speakers/beepers/piezos.

Its unlikely to design one circuit that works for every one.
My XT is different to my K6-2, and my MicroATX Super7 Board also has a different Output Level.