Platform is PC, AMD Athlon 64 3500+, dual core,1GB mem, WinXP Pro SP2, nVidia GeForce 512 MB 8500GT, DX 9.0c.
I played through SC with serious voice volume anomalies, I could never hear Sam talking and the cut scene newscasts were scratchy and unintelligible.
The voice volume can be very positional, requiring movement pan to get it. I'm using headphones.
I figured this is all going to be fixed when I load and run SCPT. Nope! Sam's voice is better but still hard to hear compared to the person He's talking to, while people talking in the next room are blasting my ears off. Cut scene newscasts are still garbled.
There was ONE instance in SC that the voices were all normal.
I've tried all the sound options and settings and various levels of HW acceleration to no avail.
Anyone got ideas to try?

What kind of sound card are you using?
Are your drivers up to date for the sound card?
Are you using regular phono jack headphones or some fancy USB headphones?
Turn off the hardware mixing.

My guess is that you have some sound card that is doin a crappy job at emulating EAX. Ontop of that if you are using a USB headphones you will have even worse sound problems.