How to add silence to an audio file with Audacity

Step1: To add silence to an audio file first open your audio file in Audacity. Once the file is loaded,
click a position on the audio waveform where you would like   the silence. From the menu bar, click GENERATE and click Silence.

Step2: From the Silence dialog box enter a desired Duration for the silence and click OK.

Step3: This will add the duration of silence as specified within the position selected on the audio waveform.
To save, click FILE > EXPORT > EXPORT as.

Credit : https://help.cobrafiringsystems.com/hc/en-us/articles/5643750980251-How-to-add-silence-to-an-audio-file-with-Audacity