Prevent phone from automatically lowering media volume
I'd say this is solvable with an automation tool like Tasker, Llama &amp; Co. Rawly described profile:

Condition: Headphones plugged
Action: Set audio level to X

Where "audio level" in many cases (at least with Tasker) can be defined more specific (e.g. different levels for notification, alert, incoming call...) -- and "X" being the specific level you'd like it to have.

This done, everytime you plug your headphones in, you'll have the same media volume you've defined.