Unlike other file formats like pdf, djvu, webm etc, while transcluding a transcribed audio of mp3 format on Wikisource, the source tab is not showing up between the standard "Page" and "Discussion" tabs at the top.
For example - The source of this NS:0 transcluded page is this this mp3 index page on Bengali Wikisource, which should show up in the Source (উৎস) tab between the Page (পাতা) and Discussion (আলোচনা) tabs.