How to enable the "Stereo to Mono Mixer" feature?

Single Sided Deafness Solution

The default "Stereo Mode" sends the (L) audio signal to the Left ear and (R) audio signal to the Right ear. For those with single sided hearing impairment, turn on "Mono Mode" which sends the (L)+(R) combined audio signals to both the Left and Right ear ensuring full audio no matter which ear is hard of hearing.

While the headphones are in use (i.e. playing music), press & hold both “+” and “play/pause” buttons simultaneously until you hear a voice prompt “Mono Mode” / “Stereo Mode”.