According to Yahoo Movies on Sunday, director of "Guardians of the Galaxy" James Gunn explained the emphasis that the movie soundtrack would have on the story line. Both a commercial and orchestral soundtrack exist in regards to the score of the film, some of which involve some kind of 70s retro set of tracks. The orchestral version is by composer Tyler Bates.
Director Gunn explained that he worked with Bates by partially composing some of the music and also playing them on set. This allowed a better method of immersion for the actors, as they would be able to understand the feel of certain scenes as they were being filmed. Some music were key for emotional as well as action sequences. Both the musical score and soundtrack made a greater impact on the actors' performance abilities. Also, minimal spoilers were dropped about "Guardians of the Galaxy" as Gunn explained their methods.
"One of the main story points in the movie is that Quill has this compilation tape [Awesome Mix Vol. 1] that he got from his mother before she died that she made for him. It was of songs, that she loved, all songs from the 1970s, and that's the only thing he has left of his mother and that's the only thing he has left of his home on Earth. He uses that as a connection to his past and to the sadness that he feels of having left all that and lost all that."
This could possibly explain his orneriness when confronting the alien guard who had confiscated his personal items. Obviously his emotional attachment to such a device and his only connection to his mother were met with drastic, electrical consequences. It appears to be a rather recognizable vintage cassette player with the foam, orange colored head sets. It's also seen being worn by Gamora as she is about to seemingly kiss Star-Lord. That being said, there appears to be some more emotional tie-ins to the device that plays the "Guardians of the Galaxy" soundtrack.
It had been explained in the past the Gunn did want to ground such an alien environment by using Earth's recognizably familiar songs. Also, Marvel and Hollywood Records will be releasing three albums from the film come a few days prior to the movie's release date.
That being said, "Guardians of the Galaxy" is set to be released come Aug. 1.