NAudio includes a fully managed A law encoder, in the ALawEncoder class, allowing you to pass in 16 bit samples and get out 8 bit a-law encoded bytes.
Alternatively, the simplest way to convert files would be to pass your WaveFileReader into a WaveFormatConversionStream (with target format of WaveFormat.CreateALawFormat(...)) and then pass that into WaveFileWriter.CreateWaveFile