We always end up telling someone to type volume xx in console during an HLTV session. I use this simple script to adjust volume in hl mods. It's trivial, but I've done the tedious part for you:

alias vol10max "echo Max volume level;"
alias vol10l10 "volume 1 ;echo Volume level: 100; alias vol10up vol10max; alias vol10down vol10l9"
alias vol10l9 "volume 0.9;echo Volume level: 90; alias vol10up vol10l10; alias vol10down vol10l8"
alias vol10l8 "volume 0.8;echo Volume level: 80; alias vol10up vol10l9; alias vol10down vol10l7"
alias vol10l7 "volume 0.7;echo Volume level: 70; alias vol10up vol10l8; alias vol10down vol10l6"
alias vol10l6 "volume 0.6;echo Volume level: 60; alias vol10up vol10l7; alias vol10down vol10l5"
alias vol10l5 "volume 0.5;echo Volume level: 50; alias vol10up vol10l6; alias vol10down vol10l4"
alias vol10l4 "volume 0.4;echo Volume level: 40; alias vol10up vol10l5; alias vol10down vol10l3"
alias vol10l3 "volume 0.3;echo Volume level: 30; alias vol10up vol10l4; alias vol10down vol10l2"
alias vol10l2 "volume 0.2;echo Volume level: 20; alias vol10up vol10l3; alias vol10down vol10l1"
alias vol10l1 "volume 0.1;echo Volume level: 10; alias vol10up vol10l2; alias vol10down vol10l0"
alias vol10l0 "volume 0.0;echo Volume level: 0; alias vol10up vol10l1; alias vol10down vol10min"
alias vol10min "echo Min volume level;"

echo "Using 10-level volume control"
alias volumeup vol10up
alias volumedown vol10down

You need to run the alias at a default volume to start at and bind some keys to volumeup and volumedown. I have this:

echo "Defaulting to volume level .2"
vol10l2

bind "PGUP" "volumeup"
bind "PGDN" "volumedown"