Not having enough sleep affects on your mind in (somehow) funny way. For examle, when i don't sleep for just one night, i can hear voices in riding train's noise. They usually are like my friends' voices, but not always. Also, sometimes you can even hear music this way (i once did, it was some pretty good hardrock or industrial, but i couldn't hear voices clearly). However, hearing non-existing sounds is usually bad thing, so if it happens to you, check your condition and fix it, if possible.