In the function description it says:
_this select 3 (Optional): BOOL or STRING or OBJECT or CODE - true to force radio, name of custom radio channel, listener's channel, returned value of code
Some months ago setting this param to false would force radio off as it does when using kbTell, as in the speakers would speak through radio only if enough distance was between them.
Now it doesn't work anymore, the speakers will always speak through the radio no matter what.
If you set the channel to "direct" it of course forced radio off but then the speakers will not hear each other as distance grows..