Ok guys, i've been doing some experiments on how to fix the sound problem in Corpse party, and finally found a solution to fix this problem.
1) you need UMDGEN 4.00
2) Open the Original Corpse Party ISO/Corpse Party USA ISO with UMDGEN 4.00
3) Click UMD Properties tab, on the lower left under optimize image, click Optimize
4) Go back to explorer tab and click + on PSP_GAME then open SYSDIR
5) Rename EBOOT.BIN>>>EBOOT.OLD and rename EBOOT.OLD>>>EBOOT.BIN
6) Click Save and choose .cso
7) Your gonna need sonic stage installed and sound forge
8) Set your JPCSP configuration, under media tab, check the"use media engine", "extract media files to tmp folder" and "decode audio files with sonic stage"
9) Open JPCSP and play the game and close it (so that it will make a tmp folder)
10) Open sound forge
11) Go to file and click open, and find the file of Corpse Party (jpcsp-windows-x86/tmp/NPUH10117/mpeg-39641088)
12) Select ExtAudio and open
13) Go to file and choose save as,choose save as type to .wav
14) Save it in same folder with the original ExtAudio
15)IMPORTANT: Enjoy playing Corpse Party with full audio/sounds and voices