Purchasing BioShock Remastered usually adds the "BioShock (Original)" to your library as a separate entry.

Known for DRM-free games, GOG.com often bundles the original and remastered versions together.

Finding a legal "free download" for the original BioShock (2007) is rare, as the game is a paid title. However, it is frequently bundled or offered as a bonus on major platforms. Where to Get BioShock (Original)

Enable "Stereo Mix" in your Windows Sound Settings (Recording tab). If it's missing, right-click and select "Show Disabled Devices."