OK, had a few minutes to spare so have made a SampleToWaveProvider24. It's in the latest NAudio source. Convert your 32 bit PCM file into 24 like this:
using (var reader = new WaveFileReader(input))
{
var sp = reader.ToSampleProvider();
var wp24 = new SampleToWaveProvider24(sp);
WaveFileWriter.CreateWaveFile(output, wp24);
}