sound plays too fast (slightly)

This problem suddenly appeared, and is not only very annoying, it turned out to be difficult to find the solution as well. My integrated sound chip (run lspci | grep audio):
00:1f.5 Multimedia audio controller: Intel Corporation 82801EB/ER (ICH5/ICH5R) AC'97 Audio Controller (rev 02)

dmesg | grep intel8 shows strange values for the ac97 clock:
intel8x0_measure_ac97_clock: measured 50217 usecs
intel8x0: clocking to 49828
and these values are not constant, after the next boot:
intel8x0_measure_ac97_clock: measured 50871 usecs
intel8x0: clocking to 50368

A solution: add (as root) the following line to /etc/modprobe.conf:

options snd-intel8x0 ac97_clock=48000

You can then reboot, or remove and reload the module:
rmmod snd-intel8x0
modprobe snd-intel8x0 ac97_clock=48000

You may also want to look at this tip: re-enabling sound.