phone battery usage at various audio volumes
First, you would want to measure the load the stereo presents to the phone. Headphones/speakers present a lower impedance (tens of ohms) than an audio input jack of a stereo/TV.

Any battery savings will be minimal when connected to a higher impedance load (hundreds or thousands of ohms). You also may not be able to measure this impedance with a multimeter since there may be a DC blocking capacitor on your stereo's input.

tl;dr: Any power savings are negligible.