You get the vanilla version of the game, the one from the old page of GoG, with the OLD INSTALLER. The 1.0.0.7. You install it alone without patches. You play only through the game's own soundtrack. After beating one song, if the weapon it gives you is not rare or bomb/invincibility, you don't press continue button, but hit the escape keyboard button; then you press NO. Upon return, the weapon rewarded to you has changed.

It takes quite a few times until a Furia/Rare weapon appears. And collectibles like the bomb and star are much more rarer than the rare weapons. By playing through symphony's ost, you should end up with 1 copy of all 8 rare weapons, and all 8 normal ones, a bomb, a star, and 3 random songs.

Uninstall the game, choose to KEEP your saved game. Then, install the modern, GoG version of Symphony, and apply the patch normally. Upon entering the game, you have all your rewards intact...

The original vanilla version can be acquired through 2 different means:

1) You bought it back in the day and backed it up then (2012)
2) You go to... Forget it, i can't say.