audio jack ground to main earth ground, safe or not?
It should be safe, as sometimes devices such as amplifiers without earth/ground that have only 2-prong mains plug can anyway be connected to other devices that are earth-grounded with a 3-prong mains plug.
Even if none of the devices are earth-grounded via mains plug so they are all floating, it can still be worth it to have extra potential-equalizing cable between devices, such as extra RCA cables. It keeps them at same potential, so it is slightly safer to disconnect and reconnect other cables, while the devices are connected to mains.
Nornally devices with 2-prong mains cables should be disconnected from mains while making other connections, to prevent damage from voltage potentials equalizing via RCA center pin or HDMI data pins. For this reason, even device manuals suggest disconnecting devices before making other connections.