...

  <profiles>
    <profile name="default">
	<variables>                                                                                                                                             
		<variable name="mycustomvar" value="customvalue">
	</variables>
  </profiles>

Since version 1.10.8 variables can also be set using the conference "getvar" and "setvar" api commands. and also any channel variables prefixed with "confvar_" will be converted into conference variables.

Sound files

Just about any format is supported, but currently it must be at the sample rate of the conference (no resampling is done). Since disk is cheaper than CPU, use a wav.

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