Lots of computers I've used allow audio gain to be set above unity, with noticeable distortion as a result. Next time you're in alsamixer on Linux, look at the fourth line down on the top which says "dB gain". It's not uncommon for it to go all the way up to +6 dB or +12 dB, which will almost certainly distort with common source material.

This happens both on my desktop Linux system and my Linux laptop.