Hi.
1/ First thing to do is to remove the back cover of the projector. Connect a external loud speaker direct to the two pins on the edge of the audio amplifier and try to project a sound film again. If the sound comes on then it is the din speaker socket on the back cover that has gone duff. Replace socket and then the back cover.
2/ If the above does not work it could be the multi switch sited at the other end of the rear record button. It’s easy to see if this is playing up, just run the projector without a film threaded and with sound full up. Press the record switch in and out and if you can hear a click sound from the loud speaker the amp is working. The fault will then be the multi switch. If this is the case you can usually get away with using some switch cleaner within it. Once squirted put the projector main control in reverse and press the record button in and out 20 or so times to shift dust etc.
3/ Last resort! I have repaired one Eumig where the large capacitor at the rear edge of the amplifier gave up the ghost. This caused no sound from the loud speaker so was a quick job to remove & test then replace. After that a repair could be more involved and possibly a transistor at fault.
Hope the above is of help.