Headphone monitoring volume very low

I am using AUDACITY 2.4.1 with a Scarlett Solo Studio on a MacBook running High Sierra. I am a total novice as far as audio goes. I have recorded a couple of test tracks of a vocalist, my wife, with headphones plugged into the monitor socket but she can hardly hear anything. Most grateful for any advice in easily digestible form.

Are you plugging into the headphone jack on the Scarlet? Is the problem that the vocalist can’t hear herself while she is singing?
If so, that is not an Audacity problem.
Are you getting a good recording level in Audacity?
– Bill

I must sheepishly admit that the monitor knob on the Scarlett Solo Studio had not been turned suffucuently. The perils of being a novice. Thank you for your reply, Bill.