Few people become better teachers or speakers without feedback on their presentations. Listening to yourself explain aviation concepts to your students is one form of useful feedback. On more than one occasion, after listening to a recording of my presentation, I raised an eyebrow at what I thought at the time was an excellent explanation. It wasn't. I made a correction and was better for it. Students also benefit when they download their ground lessons into their portable audio players and listen to them at appropriate times, perhaps during lunch, while driving, or doing yard work. After all, when an MP3 audio player is half the size of a pack of gum, it's not at all difficult to carry this handy little device with you.
Consider purchasing a small digital audio recorder. And in the unlikely event they don't already have one, encourage your students to purchase a small MP3 audio player. You'll both be better for it.