Final audio size is len*len_ratio
When you have finished converting your audio data, you need to know how much of your audio buffer is valid. len*len_ratio is the size of the converted audio data in bytes. This is very similar to len_mult, however when the convert audio data is shorter than the original len_mult would be 1. len_ratio, on the other hand, would be a fractional number between 0 and 1.
Sdl.SDL_AudioCVT Class | Tao.Sdl Namespace