Casiotone 401 makes no noise but has working speakers/outputs

I bought a Casiotone 401 at a garage sale, and it desperately needs repairs. When trying to use the keyboard no sound will appear when pressing a key or when using the built in drum machine. I know it is not an issue of the speaker because I can hear a buzz when I turn it on and off, and when using the headphone jack there is also a buzz indicating that it is on. When pressing any of the preset buttons or activating the tempo counter for the drum machine the light will turn on indicating that it is working and it is selected. I have tried to use Sync out and Aux in on the keyboard in hopes of possibly connecting it to my other synthesizer (a Korg Volca Bass). I have tried to open the keyboard myself to see if I notice anything wrong but the inside is clean with minor dust inside and everything appearing to be connected correctly. The only corrosion I noticed was on the 16 pin ribbon cord that connects the keyboard bed to the motherboard but I do not think that is an issue due to the drum machine not working either.
When I bought the Keyboard at the garage sale the guy selling it said that it had been working earlier in the day but randomly had stopped when someone was playing it.
In my time of trying to find out information about the keyboard or trying to find a service manual I have had little to no luck.
I have tried contacting local stores instrument/computer repair stores but have either had no response or are unable to help me.

I would greatly appreciate if you guys would be able to help me find out what a possible problem is or what you may suggest for me to do.

There could be a zillion reasons why this no longer makes sound, and the way to figure it out is to start signal tracing. For that you will need an oscilloscope and lots of spare time.

Maybe a wire broke loose from a solder connection (those Casios typically use soldered wires rather than removable connections). If that’s the case, it could be an easy fix. But more than likely, this will require way more effort than the keyboard is worth.