Ah, that sounds like you have an 'active' buzzer which consists of an oscillator and a sounder combined. The CB2 pin on the 6520 probably doesn't output enough voltage and current combined to run that, and even if it did it would output a single fixed tone whenever it was powered. If it makes a loud continuous tone when powered, it's the wrong sort.
What you really needed was something which was just a bare high impedance sounder with no included electronics. Something like these: (This was the first example I found - similar devices are available from other suppliers).
https://www.amazon.co.uk/sourcingmap.../dp/B077GL6HMN
The key word is 'Passive' as opposed to 'Active'. 'Active' means the unit has a built in fixed tone generator circuit. 'Passive' means the unit only contains the transducer and your circuit - in your case your PET, has to provide the pulse waveform to make it produce sound.