The software mixer (software PCM mixer) only exist on the playback side of the house, as its job is to mix multiple PCM 
stream into one that can be written to a singe audio device (since we are doing sample by sample modifications for the 
mix we added in software volume controls into this path).

For capture the only gain/volume adjustments you can do depend on what the hardware codec exposes in terms of controls 
(there are not software volume controls on the capture path). You can use the mix_ctl utility to dump all of control 
groups and to make adjust to those groups.