Basically, if you change the system's sound output (for example, connect and disconnect a headphone) Minecraft will either:

• If you connected a headphone, Minecraft will continue to output sound in the previous output source (ex. speakers):
• If you disconnected them, Minecraft will be muted completely.

The only way I could make it output sounds through the headphones was to do F3+T  (reload resource packs), however, I think this isn't correct behavior at all; considering some computers aren't very fast (for example mine), it may take minutes to reload to the resource packs, which is very annoying.

This broke on a recent version as I remember be able to swap between speakers and headphones without issue. Hope it's fixed for 1.14.