Sonic Gems Collection/Hidden content
From Sonic Retro
Lost Sonic CD JP Soundtrack
It is possible to restore the JP Soundtrack to Sonic the Hedgehog CD in the US Playstation 2 version of Sonic Gems Collection by editing the ISO in a hex editor. Just open the ISO and search for BGM_US and scroll down to line 1353439A and replace the US with JP. After being saved the game will run the Japanese soundtrack.