Guf,
Just saw your post tonight. Here is what I have to offer from reading about Quickie batteries. D cell batteries will 'go out' at a 1 volt output, maybe even a little higher. They will go out before the 9 volt pack. This is good partially because if you accidentally leave the Quickie on(and who hasn't), you will have only lost 2 D cells instead of ALL the batteries. Of course you will have taken significant life out of the 9 volt as well, but it will have maybe 50 hours left.
In my opinion, a poor solder would cause MORE resistance, resulting in LESS battery drain, and of course degradation of sound because of insufficient voltage.
Lastly, it is possible that the connection of the D cell isn't good enough to make the contact it should. Maybe a weak spring, or even a little dirt can cause this. Remember, the cell is only capable of a lousy 1.5 volt or so, and just about anything can defeat it. Ever shake a flashlight and the beam gets brighter? You just made better contact allowing more electricity to reach the bulb.