Have you updated 13 to the 13.1 Update yet using an opened project's Help Menu/Updates. If not, please do that.
I am strictly an Elements Windows user but I do try to find common ground between Premiere Elements Windows and Mac. In Premiere Elements Mac, do you have Edit Menu/Preferences/Audio Hardware ASIO? If so, please explore the Input Output there.
The results from the following should be telling....
Do you have a .mov or .mp4 movie file that comes with audio in the file? If so, can you hear the audio from this file after the file is dragged to the Timeline and played back from there?
If you have video on video track 1 and create narration clip using the project's Narration Tool, can you hear the narration at Timeline playback?
I am tempted to ask you to convert your .mp3 to WAV version with iTunes, but for now I will look for your reply in the morning and then we can decide what next.
To be continued
Many thanks for taking time to reply.
Last night I tried the files on a friend's laptop - they worked.
Today I have uninstalled, downloaded a new copy of Elements 13 and done the update.
Everything is working well so I am assuming there was some sort of corruption the first time I did the update.
"Everything is working well...." sounded great.
Wonderful news. Great job of follow through with the troubleshooting to pin point the problem and correct it.