This is fixed ONLY when using the new audio mixer in 4.15
Game sounds can no longer be heard after unplugging and re-plugging back in a USB headset during a match. This also occurs when plugging a USB headset anytime during the course of the match.
1.Plug in a USB headset.
2. Launch client in the latest cooked build
3. Get into match with any hero.
4. After load in unplug USB headset.
5. Plug in USB headset
Game can no longer be heard.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-26051 in the post.