Consider the operation of the device. The diode VD1 is necessary to protect against improper power connection to the circuit. The device is powered by galvanic cells +1.5 V (supply voltage can be increased to +9 V). With increasing voltage, the radiation power of the device increases. The sound-reproducing element of the device is a ZP-1 piezo emitter (ZP-3, ZP-22) or any other, for example, from a wristwatch. Resistors R2 ... R4 and R5, R6 set the operating point of transistor VT1 and VT2. When voltage is applied to VT1, C1 is charged, a pulse from C1 briefly opens VT2, VT2 charges C2, which briefly opens VT1, and the cycle of work repeats again. The frequency of the generator depends on the values of capacitances C1 and C2 and the resistance R6. You can turn on several more emitters in series with BQ1, then the output power will increase.