use o módulo softvol, é explicado aqui:
link
# Set your DEFAULT device to the softvol plug-in
# NOT to a hardware card device
#
# The "!" means completely override the previous default
# Not just changing/adding to it.
pcm.!default {
type plug
slave.pcm "softvol"
}
# Configure softvol
pcm.softvol {
type softvol
# Send softvol's output to dmix
slave {
pcm "dmix"
# If you wanted to you could send the output to a card directly
# But in most cases it's better to send it to dmix and let
# dmix handle where to send it. You can add a whole extra section
# to configure dmix and where it sends output, but I'm
# not covering that here.
## Use Card 0 Device 0 instead of dmix
# pcm "hw:0,0"
## Use Card 2 Device 0 instead of dmix
# pcm "hw:2,0"
}
# Add a control slider in your mixer interfaces
# i.e. KMix and alsamixer
control {
name "Pre-Amp"
card <CardNumberYouWantControlToShowOn>
# i.e. card 0 or card 2
}
# Minimum dB when slider is at 0%
min_dB -5.0
# Maximum DB when slider is at 100%
max_dB 40.0
# How many levels the slider should go through
# i.e. how granular do you want your control to be
resolution 12
}