With regard to audio devices, if all equipment is unbalanced and there are no DC- connections to earth ground, equipment will generally operate hum/noise free despite large common mode voltages between earth and audio signal+/-. With balanced audio devices, all chassis are connected to earth ground and to each other via cable shields. Properly designed balanced inputs/outputs use differential signaling and "equal" impedances between internal signal commons of each device and signal +/- , rejecting common mode noise.
When interfacing unbalanced to balanced equipment, say a laptop or cell phone audio source being charged by a switching supply to a balanced input audio amplifier, the large CM voltage difference between devices often introduces hum and noise into the signal. One solution to this is using an isolation transformer, but I’ve found that the hum can still be audible, and that the best solution to completely eliminate it is additionally to earth ground the signal common coming from the unbalanced device.
I’m wondering, however, if there are cases where earth grounding the audio signal common out could cause issues and even damage equipment. If there already exists an earth ground connection internally at the power jack for example, but the audio signal is powered through bridge rectifiers, chokes, etc. then earth grounding the output signal common would force two different parts of the circuit that should be at different potentials to be at the same potential. Measuring my laptop switching supply output, for example, which has a 3-prong plug, I get a resistance of 1kOhm between the laptop chassis and earth prong. In that case tying the signal to ground directly doesn’t seem like it would be an issue, since the two circuit commons (power and audio signal) can still be allowed to be at different potentials (correct me if I’m wrong).
Not being entirely familiar with difference switching supply topologies, do most wall-warts and brick switching supplies have their DC- output isolated from earth ground/neutral (or connected through a resistance/etc.) or are there supplies that do connect these? And/or is there a foolproof way to determine if grounding the audio out signal common of a switchmode powered unbalanced device (laptop, phone, ipad, etc.) is safe and not likely to damage anything?