The membranes have probably rotted out. Time to replace the whole speaker assembly, most likely. If it's not that then it's possibly some plastic that's cracked and come loose. The '06 models were fairly terrible as far as build quality is concerned.
Yeah it might be a similar problem as on the G3 and G4 iMacs. Best disassemble it and have a look at the speaker membrane. If it‘s sticky or turned to dust, find a (modern) replacement speaker.