I don't think it's build related. It can and has been happening randomly on several quests in all builds. The only quest that I know of where the voice has always been missing is when Wacaco (sp?) calls you to start the mission to drive the guy who wants to kill a prisoner being transported.

I've played through that 3 times and her voice has been missing all 3 times.

I've never had missing audio with the Delamain one you're talking about including in 1.06.

Seems this has progressed into the Delamain Epistrophy's being glitched as well, whenever I get near one of the cabs I get a call from Delamain that I can't answer, nothing happens.

Just now I got near the Wellsprings Cab, and it doesn't progress any further nothing happens when I scan it, can't call it.
Seems the Whole Delamain questline is glitched, I didn't have any issues like the voice missing and it worked fine during the Delamain Epistrophy missions in the "day one patch" he'd call me say a cab is near etc. But now I get a call and it doesn't progress further.

I have had exactly the same symptoms with Delamain. I *think* that I have reported it with all the saves.

(I think, because the report page lacks any form of success or failure confirmation - it might as well send all the reports to /dev/null.)

The missing voice bug can happen at any time and anywhere, not just during the Delamain quest (in my playthrough, all his lines were voiced). I'm playing on 1.06 too.

I even once had a bug where all voices stopped playing. NPCs no longer said anything, no voices during phone calls, merchants have no voice, etc. I had to restart the game to get the voices back.