It has to be done in Standard View on the MC Server. It can't be done on the MC Client. If you are just running MC on one PC, and not using the Media Network, that is where you do it.
Select the video recording (you don't need to select files, just select the recording that same way you do to play it in MC), right click on it to see the menu, >Library Tools>Convert Format (down the bottom of the list - scroll down), click the "Convert video to audio" checkbox in the Action Window, bottom left. Choose the "MP3 Encoder" to convert to. Set the options for it as you wish. Click the Convert button.
Find the converted file in the location you specified in the converter Options.
