Audio output referenced to Vcc
If isolated

Ground can simply be thought of as a reference point from which to measure voltages. What you need to understand for this is that the 555 timer moves from 0 - Vcc, i.e. the supply voltage.

Consider when the output signal is at the supply voltage. When measuring the voltage between the two, you will find it to be 0 V. Now when the output is at 0 V (ground referenced), the output referenced to Vcc is Vcc. 

By changing from referencing the output to Vcc instead of GND, all that happens is a signal inversion. From an AC analysis perspective Vcc is equivalent to GND.

This would not be suitable for mains connected things, but as it is just a battery that powers these it doesn't particularly matter. 

If connected

It will matter for the following circuitry! This could cause the supply to be directly connected to other things, so make sure you put a series capacitor after the potentiometer.

Potential reasons

One reason to choose this over referencing to ground is whether the IC is better at sourcing or sinking current. If the IC is better at sourcing current then you would connect to ground as the current will be going from that pin, through the load and to ground. If the IC is better at sinking, then connect it to the power supply as then the current will flow from Vcc through the load to the pin.

Another reason could be that it simplifies the schematic drawing/PCB layout.

(There is a slight difference between circuits as there is a capacitor from the output to ground when referenced to Vcc, and I'll be quietly honest I'm not sure why that is included.)