[PS2] Replace songs on *.AD files

[SmashingPants67]

Hi, I've bought Shin Bakusou Dekotora Densetsu, and I'm working on a hack that replaces some songs from the ones I have on Bakusou Dekotora Densetsu 2: Otoko Jinsei Yume Ichiro soundtrack CD (both I've bought from Amazon), as a test, I've replaced "男一匹夢街道" song with "恋ゆらり" song, but when I start emulation and go test in songs menu the game freezes and the cursor simply flashes between the standard arrow and the standard arrow with the Windows Vista-10 blue circle. Both original and modified file have the same exact size, but when I put in the ISO file are some differences between the ISO size and the size in DVD drive (more than 20 MB), I've checked all files in the songs directory on ISO and same directory in the DVD drive and there are no difference in sizes in both "songs" directory.
PS: Since I've not found a specific tool for *.AD audio files so I've used MFAudio to convert the song.

[amiga1200]

i can't speak for the format, but most ps2 games come with a basic form of copy-protection, namely by the TOC document.
it's a full list of the game files, a footprint if you will.
that's not counting any other possible routines on the ps2 game running in the background.
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oh, it's not as simple to cut-n-paste replacements for the most part.
the audio file (.ad) is custom and proprietary in some way, will need the encoder (codec) to create them accurately, if it's at all possible.
this is common for many consoles, sometimes it can work though.
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try playing the '.ad' files on the computer, look up it's metadata via the player that works for it, it should tell you what the audio codec is. (and with that info, it maybe findable?)
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others will have to offer pointers, too tired, apologies.