First open sonic gems iso or disc with isobuster locate the folder soniccd then open that folder then extract the file soniccd.cvm to your hard drive. Next, open up the new extracted file soniccd.cvm with isobuster and select the file you want I will use the file BGM_US.AFS as an example. Extract it using isobuster to a folder on your hard drive. Now using the tool AFS305.exe press the extract lba without names button then press the extract button. You will have a new folder called @extracted with all the afs files in there generic named like FILE0, FILE1 rename all these files with the .adx extension and run the tool ADX_Convertor.exe this will convert the adx sound file to .wav format which can be played using windows media or mfaudio etc to listen to the music. You can make .batch files to make the process simplier to convert the whole files to rename and convert the files to .wav.
Hope this helps!