Comment # 4 on bug 1182823 from Takashi Iwai

It turned out that it's no trivial task for the kernel driver; the channel map
is assigned per PCM stream, and the PCM stream on HD-audio is shared for all
different inputs, hence it cannot be specific to a certain audio route for the
built-in mic.

BTW, does the right channel sound silent?  In the past, most of problematic
built-in mics were the inverted right channel.  Then adding both left and right
became almost silence.  If one channel is silence, OTOH, it'll lead to a half
volume by summing.