In many cases we have encountered problems with audio drop-out, usually around the vocal frequencies when using extension cable on headphones with in-line remotes.
The easiest solution to this is, ironically, to add another adapter. Using the plug to socket adaptor supplied as an accessory in the Audio Technica AT338is between the extension cable and the headphone cable works like magic.
This has worked in each case we have tried, although it would pay to bring your headphones in to check it works for you. We have these kits available in our Wellington store, and note by having this kit, you will also be able to convert non-mic/control headphones to work with smartphones.