The HUD volume slider, when turned up, allows for additional audio cues that I’d say are crucial to playing the game without sight. These include cues for the KV (knockdown value) meter (showing roughly how much damage you can do before the combo automatically drops / blows out) and for receiving a bar of shadow meter.
Sightlesskombat, via eurogamer.net
Loss of hearing can affect certain frequencies more than others, so being able to control volume independently is essential.
Being able to distinguish individual sounds is particularly important when visual cues are not able to be detected as easily.
Too many different sources of information can make it difficult to focus on any of them. There is even a specific condition (auditory processing disorder) for which simultaneous sounds can be impossible to distinguish or even distressing.
Depending on what kinds of audio are important to your game, other sliders might also be useful, such as the Killer Instinct HUD slider mentioned in the quote above.
Best practice example: Killer Instinct
Best practice example: Diablo 3
Best practice example: Starcraft II