OT Stormtroopers finally explained
In the most recent issue of Star Wars insider, the question of why the troopers all have different voices when they were all cloned from Jango is asked. The response goes into the usual "they didn't know they were going to be clones in the 70s" answer, and then says the interesting thing:
"However, there is also an in-universe explanation, provided by none other than George Lucas himself. During the productions of Episode III, Lucas told crewmembers that the stormtroopers seen in Episode IV are made from multiple sources. That is, they're not all Jango clones. By that time in the saga, other clone hosts have been selected.
Lucas intimated that the selection proces has become more political than strategic in some cases - a highly placed officer's cousin might be selected over a more capable specimen, for example. The politicization results in less-than-ideal candidates, which could explain some of the embarrasing marksmanship witnessed in the original trilogy."