I've seen games that, rather than using internal volume, manipulate the system volume, which will affect other applications(including VoiceAttack). Does iRacing do this, to your knowledge?
If it does, you should be able to hear the difference when, for example, playing music or a youtube video in the background, as the volume will lower once your start the game.
If you're looking to further boost the volume, you could use
Audacity to amplify the individual sound files(up to a point).