Has the volume been good before? Here is an excerpt from a review on Amazon.com:
CONS: The TAG EATS batteries. Be sure to have a supply of AAA batteries on hand. The volume could use a boost; my daughters are sometimes holding the pen to their ears in order to hear the narrative (they're not too fond of earphones, so that's not an option for us).
If the volume was higher before, chances are that the volume control is bad. Not sure if you could get replacement parts, but you could try to contact them. Here is their support page:
http://www.leapfrog.com/en/pages/support.html
Wish there was a simpler answer for you. Good luck, thanks for using FixYa, and good luck.