increasing the rated voltage on the caps should not affect the tones generated (as that is determined by the capacitance), it only increases the safety margin in the circuit (so that the capacitors don't short out and destroy themselves from too high a voltage being applied). In general it is always safe to replace a lower voltage rated capacitor with a higher rated one.
The low volume could also be the result of the resistors having drifted out of spec. Older electronics often used "carbon composition" resistor, which were less expensive than metal film or carbon film resistors, but tend to absorb moisture over time and rise in value. I know that the schematic says to use metal film resistors, but it's a hard learned truth that what's on the schematic, and what is actually soldered onto the board in front of you, can easily be two very different things.
Finally, it could be the transistors, but doing anything more than a rudimentary check of the transistors may require equipment that you don't have. The simple check for the transistors (these are NPN, because the arrow points away from the base) is to use a multimeter in diode test mode, put the red probe on the base, and the black probe on the collector or emitter pin. Measure the diode voltage drop from base to emitter, and from base to collector. Both should be somewhere between 0.5 V and 1.0 V. This doesn't really tell you if the transistors are working as expected, but it does tell you that they aren't blown.
Feel free to skip the transistor check, however, because you say that the device is making chirp sounds, which indicates that the transistors are working. Concentrate on the caps and resistors.