An SI file is an archive saved in the Sound Interleave (SI) format, which is a variation of the Resource Interchange Format (RIF). It contains game assets used by LEGO Island, a LEGO-themed action-adventure game for Windows released in 1997. SI files typically store .WAV music files and .BMP graphics files.

You most likely will only encounter SI files if you own a copy of LEGO Island and are looking to extract the files from the installation directory of the game. SI files are not meant to be opened by a user, instead, the archive is referenced by the game to play music and show graphics during gameplay. However, some users enjoy modifying gameplay and want access to assets featured in the game. Several programs allow users to extract files from LEGO Island SI archives, including LEGO Island Rebuilder and LIME (LEGO Island Music Extractor).