This file defines the volume for various game situations.
The first column consists of the identifiers of various game situations where sound files are used like narrator speech for example.
Second column is filled by volume in % probably.
In BG2 ToB we have:

2DA V1.0
100
            VOLUME
NARRATIO    100
AREA_AMB    100
ACTIONS     65
SWINGS      59
CASTING     95
GUI         80
DIALOG      100
CHARACT0    100
CHARACT1    100
CHARACT2    100
CHARACT3    100
CHARACT4    100
CHARACT5    100
MONSTER     100
HITS        62
MISSILE     70
AMBIENTL    100
AMBIENTN    100
WALKINGC    25
WALKINGM    25
ARMOR       15