Audacity

1. Open Audio File in Audacity
2. 'Split' the stero file, so you can edit a single channel
3. Select the section of the channel you want to insert silence into
4. Save Auadicty project
5. Export audio file

Select the section and channel where you want to insert silence into.
In the below example - I have selected the Left Channel at the 2 second mark.

in below example the silence added was 1.000 seconds.

Save project an export (File → Export) audio!