The lovely "Cassette" Recorder from the I had one and I literally recorded everything I could. I remember trying to tape favorite songs from the radio and getting so aggravated when the deejay would begin talking minutes before the song ended!!!
This is the recorder Peter always kept by his side in the movie. It represents his past, and how he always carries it everywhere with him, whether it be physically when he listens to the music, or symbolically when his actions are influenced by his childhood.