Here is an example:
class mine1
{
name = "";
sound[] = {"\sound\gravel_L.ogg", db-10, 1.0};
titles[] = {};
};
the field that has the db number in it is the one. Set it to + increase the volume. Remember decibels is a logrithmic scale so that large changes might be necessary to have a significant effect.
I have also seen that field expressed without the db, just as a number. I guess that this is a scalling factor of the sound volume <1 for quieter, >1 for louder.
bored_union is absolutley right.