Reminds me of a somewhat similar experience I had when I was younger.
I used to take drum lessons, and my instructor would play his bass along with me at the end of each session. We were talking while he had the amplifier turned on, and all of a sudden strange "voices" started coming through the amp. It was really freaky - screams, moans, and some sort of distorted language. We joked about it being ghosts, and later found out that murders had taken place in the building many years earlier. I always recorded my lessons on tape, and I played the tape for my family and friends. It was fun seeing the look of shock on their faces when they heard it. I wish I could find that tape to listen to it again........I still don't know what the sounds were or where they came from, and it only happened that one time.