Answer to these questions lie in the way audio was inserted in PowerPoint. There are two methods of inserting audio files in PowerPoint presentations. Either you can embed audio in PowerPoint itself or link the audio file.
As per Microsoft
"Main differences between embedded (embedded object: Information (object) contained in a source file and inserted into a destination file. Once embedded, the object becomes part of the destination file. Changes you make to the embedded object are reflected in the destination file.) sounds (objects) (object: A table, chart, graphic, equation, or other form of information. Objects created in one application, for example spreadsheets, and linked or embedded in another application are OLE objects.) and linked (linked object: An object that is created in a source file and inserted into a destination file, while maintaining a connection between the two files. The linked object in the destination file can be updated when the source file is updated.) sounds are where they are stored and how you update them after you put them in your presentation. Embedded files are stored within the presentation, and linked files are stored outside the presentation."
In case of linked files, PowerPoint always play audio from the linked location of file( say "c:\my Documents\test.wmv"). When you play PowerPoint file having linked audio on some other machine, PowerPoint file tries to locate and play audio file from linked path i.e. "c:\my Documents\test.wmv". As there is no such audio file at the path specified on new machine so audio fails to play. Embedded audio file is contained in PowerPoint itself so audio will play on online presentation sharing websites or on other computers.
Audio file of size more than 50 MB will always be linked. You may have to set audio file size from advance options so that audio file below 50 MB is always embedded. Follow steps below:
See videos on how to insert audio in PowerPoint file.
You can visit links below to understand difference between linked and embedded audio: