Alone In The Dark

[crunchyfrog]

Think I know the answer here...
As I understand it, you get the SFX alright, you're just missing the voice overs.
The SFX data is stored somewhere within the general program data. The voice over parts are stored separately as audio data. This means that if the part of the disc where the audio (voice) data is either corrupt, dirty or whatever, it will not read it. Therefore, the game will keep running okay (you will see the mouths move), but no sound will be played as the PSX will still be looking for the audio data.
Have had this problem myself with a couple of other uncared-for originals which gave similar results.

So, clean the CD-Writer's lens, the original disc and try making another copy.

Good luck!