The loudspeaker assembly pushes down on the connecting ribbon cable between the bottom circuity (USB board) and motherboard. If your device was charging right after your reassembly, then the cable is most likely not damaged.
However, due to the missing loudspeaker assembly, it most likely came loose. The best fix here would be to source a new loudspeaker assembly and replace it.
There are a couple other DIY options, though. One would be glue, silicone or other more heat-resistant non-conductive elements that would hold it in place. These DIY solutions will most likely be permanent, though.
You could also try opening it again, then taping the connector down with some Polyimide Tape and seeing how that sticks.