In case anyone else wants / needs this, it's not impossible, you'll need the "Yu-Gi-Oh! File Extractor" which can be found here Yu Gi Oh! Power of Chaos Files Extractor (
http://yugiohextractor.sourceforge.net) which will work on Yu-Gi-Oh Online and Yu-Gi-Oh Online Duel Evolution aka Yu-Gi-Oh Online 2. Opening Sound.dat will extract all of the game's music in .wav format.
If you want to OST for Yu-Gi-Oh! Online Duel Accelerator aka Yu-Gi-Oh Online 3, you'll have to use the QuickBMS tool (Luigi Auriemma (
http://aluigi.org/papers.htm#quickbms)) with the zip.bms script file (
http://aluigi.org/papers/bms/zip.bms) on the various .ypk files. The original music files from this game with be in 128kbps .ogg format with the sound effects in .wav