The opening of a song – the material preceding the first verse or chorus – is usually called the introduction, sometimes abbreviated to ‘intro.’ The introduction is often a strictly instrumental version of music from elsewhere in the song, for instance the verse or the chorus. In ‘Midnight Special’, it is a version of the chorus.
Confirm the connection between the introduction and the chorus by singing or saying the lyrics of the chorus (‘Let the Midnight Special shine the light on me / Let the Midnight Special shine the ever-loving light on me’) in time with the music of the introduction (refer to the opening of your recording of ‘Midnight Special’). Simply start the words the moment you hear the harmonica begin.
In other pieces, the introduction takes the form of a vamp: a short piece of opening material that can be repeated as desired. This is the case at the beginning of ‘Suspicious Minds’: in a live performance, the backing band can simply repeat the same two seconds of music until the vocalist is ready to come in. The vamp can also be built on material from another part of the song: in ‘Suspicious Minds’, it turns out to be the music accompanying the start of the verse. Listen again to your recording of ‘Suspicious Minds’ if you would like to confirm this.
The music heard four times at the beginning of ‘Hang with Me’ (just after the swell in the bass) could perhaps also be called a vamp, but here the designation is a little less clear, since the same musical material accompanies the entirety of the first and second verses. Like many pieces influenced by electronic dance music, ‘Hang with Me’ makes use of repeating musical patterns called ostinatos or sometimes (in electronic music) loops.
Listen to the introduction and the first two verses of ‘Hang with Me’, paying attention to the looped musical material.