Been dealing with this issue since getting new iPhone 6 a few weeks ago. None of the settings has worked for me. The issue seems to be in the use of headphones that have a toggle feature built into the headphone that allows you to control the play/pause settings on the phone. I currently use Skull headphones and any slight movement around the phone jack (without pushing the button) creates an issue. Anything from the speed of voice, automatically turning on voice control and the static on the line all appear to originate from the interface with the headphone.
I switched to another set of headphones that don't have this feature and I can swing the headphones all day and doesn't replicate any of these other issues.
This appears to be more of a hard wear issue and not in the IOS. Maybe they can turn off this control interface but none of the current setting options address this issue. Upgrading to a new set of headphones rather than waiting for Apple because theee are the main features I want from my phone other than phone, web, and business apps.
I really have found the quality of Apple products to be getting worse over time. I should get compensated with free iPhone bucks for the cost of the new headphones