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With VLC Player https://www.videolan.org/vlc/
Step 1: Launch the VLC Media Player app on your system and open Media > Open Multiple Files.
Step 2: An Open Media pop-up window will open from where click on the +Add button to browse and load the video files to be merged
Step 3: Next, enable the check-box at Show more options to open the list of additional features. Check the box at Play Another Media synchronously ( Extra Audio file). Click on the Browse button at Extra media tab that will again open a new Open Media window where click on the +Add button and add the local audio file to be merged. Click on the Select button to choose the file.
Step 4: Next, open the drop-down menu at the Play tab and select the Convert option. From the pop-up window, select enable the Convert option under the Settings section. Choose the desired target format from the Profile tab. Finally, click on the Start button to merge the added video and audio files.