Its not so much integrated with nwn.
My players download a custom application I wrote.
It downloads mp3's from my website - which contain the voices / audio files to play during conversation or just sound effects etc
Since MP3's are smaller in size than wavs' - its a good form of compression for downloads.
When the download is completed - the same application extracts ffmpeg from itself - and then calls ffmpeg on command line to tell it to convert the mp3 file into a wav file.
This is the command line arguments I use
string args = "-i \"" + FullPathToFile + originalfile + "\" " + "-y -ac 1 \"" + FullPathToFile + sName + ".wav\"";
I do this in C# so you just need to take into consideration that some of that line are variable names, and the whole line is being supplied to ffmpeg.exe
This converts mp3 to wav
Note: 45 seconds of wav can be about 3-4 mb.
the same file as an mp3 can be as little as 200-300 kb
mp3 is more ideal for distributing audio files than just as a the plain wav files