This is a follow-up to d5390c7, where we switched to the 'PCM' mixer controls
on Sound Blaster Audigy devices, since the ones we used before only work in the
stereo-to-all-speakers mirroring mode.
It turns out we do use that mode after all, but only in a couple of profiles.
Since returning those profiles to the default mixer path yields uneven volume
steps similar to #2934, we instead add a new mixer path just for that mode.
