Attempting to render a video to a custom location (anywhere that isn't the default location) will come up with a "ffmpeg isn't installed" error. Rendering using the default name and location works, however.
AMD Phenom II Quad Core 955, 3.20GHz
8GB RAM
x64 bit Windows 7 Ultimate (6.1, Build 7601)
Nvidia Geforce GTX 960
Minecraft 1.8
Minecraft Forge 11.14.4.1563
Only the replay mod installed