Exporting audio files from GarageBand to ACX for Audible

I recorded audio files for an Audiobook and want to submit them to ACX for production, but they keep saying the RMS is too low, i.e. the volume needs to be increased. So I went back to GarageBand and edited one of the chapters, increased the volume 6 dB even though they'd only asked for minimum 3 dB. Shared it as a song to disc and when I played the MP3 on my computer with iTunes it sounded the EXACT SAME as the original audio, i.e. no increase in volume at all. Needless to say, it was still rejected by ACX. I'm tearing my hair out, not sure what to do! I have been in touch with GarageBand support at Apple and their only suggestion was to entirely upgrade the OS of my Mac, which I don't want to do as that always creates more problems in the end. I'm using a MacBook Pro laptop and a Mac desktop, both with Ventura 13.1 and the latest version of GarageBand, so not too shabby. I also checked my system output and input settings and can't see a solution there. CAN ANYBODY HELP????