Sounds changing pitch on slow computers

This bug can be noticed if you play Doom 1 / Doom 2 on a 486 25 mHz computer, in some case even a 33 mHz computer can have the same problem.

What it does
What happens is that when a lot of things happen at once, multiple enemies attacking for instance, the sound is first played at the sound card's main frequency and then downpitched to the standard pitch in the Doom engine.

Demonstration
Start Doom2 at level 30 (Icon of sin) and let 20-40 monsters spawn. As the monsters start to attack you and other monsters are in front of them then they starts to attack each other. What happens is that the computer is having a hell updating everything around the level and that's causing this error.