Conditions controlling the volume of one's voice
If the volume of the player is low, reduce ambient noises to better be able to hear them. Additionally selecting a smaller gaming space allows for lower talkers to communicate more effectively with their peers (So long as they don’t have concerns with spatial anxiety.) If it is a matter of preference, you can offer more opportunities to use actions to describe preferences. For instance, using Power (Spell) Cards might allow the player to make selections and have that seen visually to compliment their speech. Movement of miniatures, rather than describing travel can do the same. This will reduce some of the anxiety and frustration that might occur from not being heard, or being asked to repeat oneself.
For players with low or high volumes, if the child and their caregiver are comfortable, and if the cause of volume control is due to an awareness issue, you can consider the use of a simple volume chart. This is a single sheet of paper that indicates volume levels (Whisper, Low, Group, Loud, Shouting), that help child understand where their volume. A discreet tile or dice placed on a level can let the child know either where they are in volume currently.
The primary thing to realize is that a child that is speaking at a volume may be doing so out of a necessity. Demanding the child speak up, or showing frustration, is definitely not the way to support them. Be cognizant of their needs, and focus on helping them communicate in as discreet, and comfortable a way as possible.