One of the secrets of the modern world is that the signal definitions for audio were originally defined in the 1960's and really haven't changed much since. The standards used by devices like cassette decks or radio tuners for line in and line out are 100 millivolts, peak to peak, and are specified as such so that you won't blow your amplifier by overdriving the input. The one common device that was different was the output from a turntable (remember LP records?) which often required a preamplifier or, at least a special set of input connections on the tuner/amplifier.
I mention all of this because you are a rare individual that is even curious about the possibilities of projects.
Older (early 1990's) sound cards had an amplifier on the card and I still have speakers that need it because there is no amplifier built into the speakers, themselves.
SOME, Dell OptiPlex PCs also included special hardware and drivers that would route the audio to the PC's speaker if no external speakers were plugged in; but, you now have to ram Windows Vista drivers down the PC's throat in order to make it work on 7, 8, or 10. The internal speaker normally gives you the "beep" from the BIOS just before the PC boots and is, thereafter, silent.
So, all of the audio connections on a modern PC use the old (1960's) standards for line-in and line-out and you must have an amplifier between the jack and any speaker unless your OK with the feeble, hold it up to your ear, volume. The problem is not the voltage on the speaker, it's the peak-to-peak voltage driving the speaker and 100 millivolts (1/10th of one volt) won't do very much.