Function ratecv 

pub fn ratecv(
    data: &[u8],
    width: usize,
    nchannels: usize,
    inrate: u32,
    outrate: u32,
) -> Vec<u8> 

audioop.ratecv(fragment, width, nchannels, inrate, outrate, None) with the default weights (1, 0). Only the converted bytes are returned; pydub discards the state.