I have install Thief Gold from GOG and added the TFix mod.

The videos play great, and the game looks awesome, but ir seems that Garrett is less talkative than I remember. When I played the game back in the day, when it originally came out, one of my favorite parts was Garrett's comments. He doesn't comment as much as I remember, but he does say something during game play, occassionally. Is this a known issue, or is my memory just wrong?

with TFix, he actually talks a bit more, as some remarks that weren't playing previously now do. so unless you meant the audio is too silent, yeah, your memory is deceiving you.