Usually, it's a MIDI-interfaced SMPTE box for a TimeCode Stripe.
if you drive the base of an emitter-grounded NPN, via a (say 10K) resistor from the 555, this transistor can 'short out' the tone signal in time with the 555 output.
wire a resistor, say 47 K between the tone osc output and the transistor collector.
Gated tone is available at the transistor collector.
To make it single supply, throw away the twin-t osc and replace with a phase-shift osc, based on a single transistor.
To attenuate the signal, the 47 K might do that already