This is very unlikely, Valve is notorious for using proprietary APIs... we're talking about gaming here so this is unlikely to change.
FOSS hatred, quand tu nous tiens...
Gabe has recently said that OpenGL is awesome because many Chinese people are still using Windows XP and can't access new DirectX releases and that they are looking at switching over to OpenGL for Windows, too. I am pretty sure Source 2 will be either OpenGL only or will have support on day 1.
I'm curious, other than DirectX, what proprietary APIs are they notorious for using?
Miles Sound System and Scaleform come to mind.
However, I think ever since their Linux work began, there's been a push to use more open source solutions (the obvious one being SDL),
and I'm pretty sure they've been using Speex for quite some time now for voice communication.