surely the latest kit Kat roms have it built in
No it must have the fix:
For those STILL having issues,
Check /system/lib/hw directory for the file "audio.r_submix.default.so".
If it doesn't exist, ask your device maintainer to add audio.r_submix.default to
the product_packages in the device.mk file. (You may not know what it means, but they will.)
EXAMPLE (Thanks, [MENTION=3900214]runandhide05[/MENTION]!):
http://review.teameos.org/#/c/389/
(Thanks, [MENTION=4282424]hwong96[/MENTION].)
http://forum.xda-developers.com/har...enable-mirroring-device-t2812193/post54062732Samsung Galaxy S III (as of v4) (ROM MUST HAVE APPLIED FIX, CHECK ABOVE)
I have so far only found one that has it build into an experimental version.