Found the problem - and you'd wonder why 'they' did this! I've had to split the all the files to under 100K.
'The Sound Options dialog box also indicates where the clip is stored. If it’s stored within the presentation file, the dialog box will display the location Contained In Presentation. If it’s stored in a separate linked file, it will display the file path. (PowerPoint normally stores a sound clip in a separate file if it’s larger than 100 KB. ) In the latter case, if you are going to present your slide show on another computer, you’ll need to take the linked file with you'.