Yes, you did mute the audio, but added it back to the merged clips with AudioDub(combined, the_audio). What is the colorspace of the a,b,c clips? You can find out by using Info(), so for example a.Info().
However how come you are not given an error when it doesn't work? If it doesn't work, in the sense that it will not output any video and/or audio, then it will give you an error (or should anyway). Else, what exactly is not working as it should? ("can not concat?")
Also try
Code:
combined= a+b.AssumeFPS(a.framerate)+c.AssumeFPS(a.framerate)
I guess...