Is grounding needed when using adapter to phone?
Typically, stereo headphones use 3 wires: Left, Right, and common/return, which may be called ground. I'm assuming that you are referring to the common and not a separate ground.

If you don't connect he common, what you end up with is the two source channels connected in series, but out of phase, to the two earpieces, again in series but out of phase. The result is that any signal that is common to the two channels will cancel out and not be audible while signals not in common will be combined.







simulate this circuit &ndash; Schematic created using CircuitLab

The practical result, on many pieces of music, is that the vocals, which are typically "centered" in the stereo signal will become inaudible. In fact, this is a trick that has been used in the past to remove vocals for recorded music. Unfortunately, you will also lose any instrument that is also centered.

On the other hand, if you are talking about a 4th wire, a separate ground, you cab probably disconnect this without harm.