The drama CDs are essentially the linking thread between the seiyuu who sang on stage and the stories sketched in the manga and written in the novels by creator Arisugawa Kei. Like the rest of the franchise, these were popular enough to spawn a "plus" genre, which appear to have been broadcast as "radio" at some point. (There were also radio CDs of some nature which I haven't managed to track down at this point).
The Drama CD universes have connections to the manga ones but there are some specific differences. Most importantly are changes in some character names from the original manga series - examples including the names of the boys' teacher (Mizutani-sensei instead of Shimizu-sensei) Takagi's band (Cassandra Next instead of Cassandra) and band members/associates (specifically Hiroyuki instead of Hiroshi and Rika instead of Yuka). They also do not necessarily belong to a specific sequence - although the "M Ni Makasero" CDs are numbered as such, they don't actually lead directly on from one another in most ways.
All scans of CDs are from my own CD collection, so if I'm missing a CD, I'm afraid it won't be pictured!