Bem, encontrei a resposta. O comando correto é este:
C:\Temp>ffmpeg -i test.wav -map_channel 0.0.0 -map_channel 0.0.1 first.wav -map_channel 0.0.2 -map_channel 0.0.3 second.wav -map_channel 0.0.4 -map_channel 0.0.5 third.wav -map_channel 0.0.6 -map_channel 0.0.7 fourth.wav