Using an unconnected audio splitter degrades audio signal quality?
As long as the connection is secure. The stub created by the splitter will have negligible effect. Audio frequencies (approx 200 Hz to 20 kHz) are too low to care about electrical stubs for normal audio applications.

The effects of stubs and transmission lines only needs to be accounted for when the length of the line is a non negligible compared to the signal's shortest wavelength. For audio, the shortest wave length can be calculated to be about 1500 m (see wavelength calculator). If your application required sending analog audio across 100s of meters (perhaps you are reenacting WW1) then perhaps you would be concerned with the effects of high speed transmission lines.

If you were transmitting gigabit ethernet, you'd start worrying if you line length was more than a few inches. If you were transmitting 60 Hz AC electricity across North America you might also care. In both cases, the instantaneous voltage measured at different points along the line will be different. Having a stub would cause reflections that would interfere.