ACX

ACX is an archive for multiple ADX sound files used in Persona 3 and Persona 4. You can primarily find these files (bundled with RMDs) inside PAC archives. Typically, ADX files contained in an ACX archive do not loop, and have a predetermined start and end time. It is yet unknown what controls the allowed duration of each clip. Persona games use ACX files exclusively for in-battle voice clips.

acxtool.exe
The Commandline ACXTool, created by TGE, can pack ADX files into an ACX archive. Download it adxencd.exe here.