When sending or transferring an Ableton Live file to somebody else, or submitting it for a course assignment, it is not uncommon for the Ableton Live set to be missing audio files when opened by another user. This usually occurs because the audio files are not located in the Ableton project folder, and are instead being read from somewhere else on the original user's machine. When the recipient opens the project, this leads to audio files being missing in the Live set. 

This can be remedied by creating a copy of your project, opening it, then using the Collect All and Save function. This will collect and save all external media files referenced in the open Ableton Live set to a new Live set. To do this, navigate to File > Save a Copy, then select a destination to save the copy. Then, open the copy, select File > Collect All and Save, and select all the types of files you want saved to the new Live set.

Alternatively, you can also take the following steps to manually include all referenced audio files in your Live Project:

1. Open your Ableton Live set.
2. Navigate to View > File Manager.
3. Click on Manage Project from the new window on the right.
4. On the right-hand window, expand the section for External Files.
5. Ensure the option to Collect into Project is set to Yes.
6. Click Collect and Save in the bottom right.
7. Repeat steps 2–6 until the option to Collect Into Project is no longer available, indicating that all files have been successfully saved. 
8. When successful, you will notice that the option to Collect Into Project is no longer available, as all the files have been saved.