Sounds changing pitch on slow computers

This bug can be noticed when Doom 1 / Doom 2 is played on a slow computer, for example a 25MHz 486. In some cases even a 33 MHz computer can have the same problem.

When many things happen at once (multiple enemies attacking simultaneously, for example), the sound is first played at the sound card's main frequency and then downpitched to the standard pitch in the Doom engine.

Demonstration
The effect can be demonstrated on Doom 2 MAP30 (Icon of Sin). First allow 20-40 monsters to spawn. After a short while, monster infighting is likely to begin, and the effect can be noticed.