Name: FMOD Ex
Open Source: No
Source: http://doomgod.com/ports/fmod/fmodapi42636linux64.tar.gz (cannot be accessed on the site; link suggested by ZDoom Wiki)
FMOD is a commercial audio library made by Firelight Technologies that plays music files of diverse formats on many different platforms. It is used in games and software applications to provide audio functionality. FMOD supports a range of audio formats and numerous different operating system platforms.
This is the only dependency keeping me from building GZDoom. It doesn't require any dependencies, but the problem is that it's proprietary.