Converting 3 seperate 4.5V battery operated devices to one AC power supply

No, I would not tie the grounds together for such a distance. That does make the data connection more tricky though. In your case, the two grounds shouldn't be all that different since presumably they are tied together back at the breaker box in your house. In that case you could use differential signalling, knowing that the common mode offset should stay bounded to a few volts at most.

However, I think real isolation of the data signal is a better idea. If you can use ethernet, then you don't have to do anything further since that is already isolated. Otherwise, it's probably easier to use a opto-coupler at the receiver. The transmitter would send pulses of a few mA, which is enough to light the LED in the opto at the other end.

One simple scheme is to use UARTS, with the line idle level being no current. Cheap optos can be surprisingly slow, so either get fast optos or run the UARTs at a slow baud rate. Even a really slow opto can support 9600 baud well enough, and you can go much faster with the right choice of opto. If you need high data rate, then ethernet starts to sound good.

You could indeed! But first things first:

• determine each device's power consumption and sum them all up
• make sure your laptop PSU can supply at least this much

Things to watch out for:

• Even though they may run on the same voltage, the Rpi and Arduino might not get along. Noise and ripple from one device will be superimposed on its supply lines, so to save you the hours of debugging I would STRONGLY suggest separate supplies for anything with a clock.
• Inrush currents may be several times greater than average currents (big current spikes are common when charging up lots of parallel capacitors on each board at initial power-on) so if you don't have the inrush current spec available from the device's datasheet, just double (or triple) the average/rated current to be conservative.
• If you use linear regulators, you won't have any extra clock frequencies to worry about, but you will have lots of heat. Try to find a regulator such as the 7805 which has its tab connected to ground. That will make it much easier to sink heat to a metal enclosure, if you end up putting the completed project in one.
• Aim for "star" grounding such that all your individual device grounds come to a common point which is as close as possible to the lowest-impedance ground reference. This might be the negative return on the laptop supply.
• Your audio amp very likely used bipolar supply voltages. If you don't have a true negative supply available this could be a problem. It might be simpler to find an audio chipamp such as an LM386 that runs off a single supply than to generate a negative supply locally.