By: Charlie Reynolds
R1 value = 200k ohms
https://www.multisim.com/content/RvNvTAgawFRdQr2hfDzjjf/555-monostable-multivibrator/open/
I found this circuit that utilizes a 555 timer operating in astable multivibrator mode. The astable multivibrator mode creates “free running waveforms which can be adjusted”. Using resistors in series, the different buttons can use different amounts of cumulative resistance to create 5 different tones.
Charlie Reynolds Analog Lab Blog 