One of the easiest types of mods to start with is changing the BGM. In Persona 5 (PS3), the music can be found in ps3.cpk/sound, where there is a pair of ACB/AWB files named bgm.
To edit these, you’ll want a program called ACBEditor. You can unpack all of the ADX files contained within the AWB by dragging the ACB file onto the EXE.
When you’re done editing the contents, you can drag the folder back onto the EXE to repack the updated files back into bgm.AWB.
The individual files in Persona 5 are in .ADX, a Criware format. You can listen to these files with foobar2000 by installing the vgmstream plugin. You can create a looping ADX file from a WAV using Atomencd. This uses sample numbers to determine the start and end points of the loop.
Did you know there’s a mod to randomly shuffle the music tracks you hear during normal battles? Give this guide a read for more info!