Certainly do-able, as long as the PC PSU has heaps of grunt on the 12v rail - at least 20+ amps. You would need to be careful about wiring in your speakers to ensure that the impeadance doesn't dip too low. 4ohm speaker @ 12v = 3A, per channel at any decent volume, not including the sub. It's only napkin figures, but the current draw is where the kicker is - a car 12v system can handle quite a few more amps than the PC PSU.
I don't think there is any risk of letting out magic smoke, but you will know when the PSU is overloaded - dreadful distortion, and probably shutdown.
And if you run close to the distortion line, you will firetruck your speakers. The sine wave audio is no longer sine wave and will have a significant DC (non-sine) component. Speakers don't like DC.
That's what kills good speakers - running them from an underpowered amp that clips.
Ah, and you may want to add a farad capacitor, just in case. That will handle a few dips to the 12v line. And maybe some filtering to remove the PSU "hash" that you get from switching supplies. A few 1uf, and 0.1 uf caps across the 12v rail, maybe a choke in series with the 12v line. A few dozen turns of 12-14 AWG wire on a ferrite rod makes a reasonable choke and could help to suppress hash.
Try googling - it's been done a fair few times - try instructables web site.
My thoughts are no longer my own and is probably representative of our media-controlled government