Unknown type of audio interface/connector/cable

It's really hard to tell what that is in the photo and I am with stereo5 a tape deck would go into a preamp or integrated not a speaker.

I think what you have is a part of an 'all in one' stereo which really is a bunch of boxes that are NOT complete separate device. Rather they share one power supply and are just in separate boxes to 'look good'.
So IMO your cassette box is worthless junk unless you happen to find the rest of that particular so called 'system'.
Whomever sold it to you ripped you off (unless you bought it like at Goodwill for $5,99)

I found it in my grandma's attic, so no loss :))

This is how the cassette deck looks, it has a dolby noise reduction button on it, so it's definitely made for recording. 
It has a power outlet and that connector, whose only use i suppose would be for audio playback.

I did also find another Akai radio/amplifier/tuner with a graphic EQ which in the meantime I plugged into my computer to test and it does work perfectly.

I attached photos of both here: https://imgur.com/a/Yp9TVec .