Unless there is a good reason to have the thread SynchronizationContext preserved, all of your await ...calls throughout the library should have .ConfigureAwait(false) appended.

Not having this causes deadlocks when having to call your library in a synchronous manner such as:
var meta = ffmpeg.GetMetaDataAsync(input).GetAwaiter().GetResult();

Obviously you would ideally not call it

Create a new service: SubsConverter to help dealing with subtitles conversions (vtt, srt...)