Replace crystal earpiece in bat detector circuit
The crystal earpiece has such high impedence that it presents virtually no load to the CD4024. Normal earphones have impedence in the 16 to 32 ohm range, which would (at 9V) cause the CD4024 to provide far more current than it's capable of, potentially damaging or killing it.

You need an amplifier with high input impedence which is capable of driving headphones. The LM386 you're using already in your circuit would work. The datasheet gives an application circuit for an amplifier connected to a speaker. You've already got the 10k volume selector.