The most complete library for audio/video file reading, writing, decoding, and everything else is ffmpeg's (the library is called "libav", but there's a fork of ffmpeg called "libav" so it's really confusing; so I'll just call it "ffmpeg")
There is no other offering, free or not, that has comparable breadth of support. I have planned to c2nim it into working condition, but have not yet found the time, and even if I did, it would likely be less useful than one can expect. There are a few issues to consider about this library - 1) It wraps a lot of other libraries, such as LAME, x264, ...; ffmpeg itself is mostly LGPL, but each library has its own license (x264 is pure GPL, for example), which for some users is in conflict with Nim's less restrictive license. Furthermore, there is essentially no one to negotiate a non-free license with for the most part (x264 do, but only for their part; for the main ffmpeg lib which includes e.g. the h264 decoder, there is no way at present to get a non-free license) 2) It changes often, and as a rule of the thumb, one should expect breaking changes about 1-year after the release of each version. The required changes are usually small, occasionally big, but in general this proves hard for any kind of language wrapper to remain well supported, working and relevant. I don't think it's a coincidence that almost no language has up-to-date ffmpeg wrappers (and those that do usually wrap the .exe rather than link to the lib); it's a moving target.
