Mono playback in headphones (w/mic)

I have a pair of Sennheiser 4.40HD Bluetooth headphones. when i connect them wired to my macbook's headphone jack (through a dedicated adapter cable), the output is in mono (in both earphones).

i learned that the problem is the microphone settings in System Preferences - when i plug the headphones in, the Input settings are hijacked to the headphones' mic - go automatically from "Built-In" to "external" without the ability to change back to "Built in". is there a fix for that?