So much complaining about the restrictions, yet quite a few guys wanted this.
Anyway, I really don't see what is so limiting about not using custom sounds. All Source and Goldsource games have a decent array of sounds available, not to mention you can play around with in-game entity settings to adjust pitch and add effects to them, so really. Get creative guys...
I'll always believe what the creator of Audiosurf said:
Having complete freedom would have killed his project, even though he could have had it, he chose not to, he put limits on himself
Also, from Deus Ex:
There is no difference between complete freedom and complete anarchy.