there will be no bong sound if there is no rom fitted to U18, this is where the O.S. and check sum info is stored (among other things)
there will be no bong sound if the data cable is not connected.

the most common faults I come across on these boards are: faulty ram at U9, faulty Digital to analogue converter at U19 (equivalent to this is a DAC0808) but this will not cause an "Interface Error message".
Faulty U29 & U30.

I have also come across a faulty oscillator at osc1 once or twice but if you have a logic probe then probe at W10, if it shows a pulse then your oscillator is fine.

I have had a faulty cpu once at U13, u can swap this with the CPU chip at U4 on the CPU board to prove if it is working, they are the same.

If it gets to difficult for you I can always sort it out for you.