There is no audio effect for combining channels in Premiere Elements. However, you can create a custome output that will combine the channels when you Export & Share your movie. In Export & Share, under Devices, select Custom and click Advanced Settings. Leave the Format at H. 264 and the click the Audio tab about halfway down the panel. Under Basic Audio Settings, set Channels from Stereo to Mono. Once you save and name this preset, you can use it for any future projects.
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