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.
Tools
acxtool.exe
The Commandline ACXTool, created by TGE, can pack ADX files into an ACX archive.
Unpacking: acxtool.exe -u [file] [destination]
Packing: acxtool.exe -p [folder] [file]
Download it here.
RMDEditor
RMDEditor.exe is a program created by OfTheSeventh and maintained by ThatTrueStruggle. It features a GUI similar to BrawlBox and is capable of opening PAK/BIN, RMD, and SPR. As of the latest version (v1.4.6), listening to, exporting and replacing the ADX files in an ACX container is supported. Download RMDEditor here.