I'm thinking of an earphone used with old-fashioned pocket transistor radios. A coil next to a steel diaphragm, 1/2 inch diam. (Maybe the diaphragm is magnetized, one face north, the other face south.) Run AC through the coil. It moves the diaphragm back and forth. The sound travels to a tiny hole at the other end.
To do sonic welding, I think will require these changes:
* Multiply the power input. Make the coil able to carry a few watts.
* Mount the diaphragm so it has more flexibility to move back and forth.
* Seal around its edge, thus making it impossible for moving air on one side to cancel effects of moving air on the other side.
* The housing should be rigidly constructed so it does not vibrate. All sonic energy must be concentrated to the opening.
* Hold the opening near plastic pieces which are touching.
It could help to sweep through a range of frequencies, in case one is more effective on the physical substance being welded.