Kara Shallenberg and her 10-year-old son, Henry, exhausted the audiobook collection at their library in Oceanside, Calif., five years ago. With Henry’s appetite for listening still strong, Ms. Shallenberg began to record herself reading his favorite books. Eventually she upgraded from a using a tape deck to burning CD’s on her laptop computer. Last fall she took her hobby to a wider audience.

Ms. Shallenberg’s recordings of “The Secret Garden,” “The Tale of Peter Rabbit” and other works are now available, free, to anyone with an Internet connection and basic audio software. She is part of a core group of volunteers who give their voices and spare time to LibriVox, a project that produces audiobooks of works in the public domain.