For the HD version of P3RD it requires a different patch tool than the non HD version,, coincidentally I am also making a patch for the game,, some patches such as audio mods etc. are working well,, please use the tool that I often use to make these patches...To find the offset, you can use...
To modify PSP games, it's actually quite easy Some tools, such as QuickBMS, can be used to extract or repackage,,To modify the audio, you can use at3tool for encoding to attract3,And ffmpeg to create PCM WAV from MP3, etc
for some games it might be possible, like the Monster Hunter series, etc., GTA series, etc. That's possible...but the application of the texture is different from what happens in the emulator...for example, Monster Hunter Portable 3 can almost change some of its textures, or change the BGM...