Seems strange, but if there really is a change in volume, and it's not just a kind of psychological effect of knowing that you had the keyboard apart and are now fearing that you damaged something, then there are a few things that come to mind.
Start with the easy things, first. Check the main settings. Did you reset the keyboard? If so, this could have changed the MasterEQ setting, and that could definitely affect the sound. This also could have changed the touch sensitivity setting, which could change the volume of your playing based on how hard you're hitting the keys.
Otherwise, I wouldn't think that it would be keyboard contact related, as those are either on or off and don't vary volume with any sort of resistance, to the best of my knowledge. However, a very long shot, esoteric possibility that comes to mind could also involve the velocity sensitivity. As I understand it, the velocity sensitivity is accomplished by measuring the minuscule amount of time it takes the key to go from triggering one set of contacts to another as the key is pressed. The shorter the time, the faster/harder the key is being hit, and the louder the sound (when touch response is active). Maybe residue on the contacts could be causing a very slight delay for the keyboard to fully recognize that the second set of contacts has been engaged, making it think you did not hit the key as hard as you did? Like I said, seems quite unlikely, and seems very unlikely that it would affect all keys the same way, but maybe a possibility. To check this, I would just turn off touch sensitivity in the settings. If you notice that the keyboard gets noticeably louder than even your fastest/hardest key presses with touch sensitivity on, then maybe it does have something to do with the velocity sensitivity -- even if it is not the exact scenario that I described.
Maybe something else got disturbed when the keyboard was apart. Still, I would think if it was a speaker/audio connection, it would either sound or it would not -- or at least, with a loose connection, you'd get a broken-up sound or static. I wouldn't think it would just reduce the volume of the sound.
