The lowest pitch humans can generally hear is around 20hz. MP3 standards filter out anything under 20hz or over 20Khz. And I can't imagine a cheap built in microphone that picks up anything anywhere near that low. So I seriously doubt your MP3 recorder can capture sounds lower than you can normally hear... Plus, as Michael asked, what do you expect to hear when you play it back? If you can export the file to a computer and examine it in some recording software, you can look at what frequencies are there. I'll be surprised if there is anything under 50hz.