Activision has been licensing Vietnam War-era music for Call of Duty 7, according to a "reliable source" on That Videogame Blog.
Music from Cuba, Africa and the Soviet Union is also mentioned, suggesting a Cold War setting for the seventh instalment.
The involvement of Activision/Treyarch lead producer Noah Heller is also noted. He said last year that he was "happy that we put [WWII] to bed" with Call of Duty: World at War, which adds weight to these claims.
Plus, with Infinity Ward occupying the modern-day battlefield with Modern Warfare 2 (Call of Duty 6), the choice of available conflicts is narrowed further still.
Modern Warfare 2 will be released for PC, PS3 and Xbox 360 on 10th November.
Our Modern Warfare 2 gamepage has more information.