Chris, I've just checked omxplayerGUI on my Pi 4 and it is playing an .m4a audio file fine (I don't have any .mp3 files on this SSD, but it was also playing them last time I checked).
On the bottom right corner of the omxplayerGUI window is a slider. It doesan;t have a name, and usually the control button appears to be almost all the way to the right (but the number above it is usually '0'.
Try grabbing that and sliding it a little further to the right. It doesn;t go very far, but it makes a huge difference if you have any audio happening at all. That 'might' solve the problem. It seems to work like the 'decibel gain' function in ALSA Mixer.
If that slider does nothign to help, it is possible you might have one of the omxplayerGUI settings changed.
Have you read the manua?
You already have a copy of it, but most people never bother to read it because it is 48 pages of how to get the best out of the program - and they find that a bit boring. And no, I'm not being sarcastic. As a tech I have been sorting problems for people who didn't get around to opening a manual for years. In fact I'm still doing it with things like friends' washing machines. And worst of all, I'm guilty of it myself
However Guenter went to a lot of trouble to try to cover as much information as possible in that manual. Including pictures. You might find the answer in there.
Also,if you open the 'kwebhelper and omxplayerGUI settings', a little over half way down the page, there's a spot where omxplayer can be told toi use VLC to play Audio for example. That is just one of a number of settings that can be changed in omxplayerGUI.
Anyway, it might be useful to have a look through the omxplayerGUI section of the settings page, and to check them against info in the manual. If the sound is working in VLC, it should be working in omxplayerGUI.
If it is not working in omxplayerGUI, I would suspect one of those settings.
I'm assuming you are not running headless.