Thanks to the Nights Into Dreams website we are able to see an early video of the game, when the beta had different graphic details and different level layout in Spring Valley. A more recent beta was shown at E3 1996 with some different sound effects and item placement. In january 2009 the E3 beta version (that is the same build of the Japanese NiGHTS Special Sample disc) was somehow leaked online and you can donwload it from the X-Cult mirror! Thanks to MalanTp that has created some videos to compare the beta to the final game, we are able to check all those differences. You can download some hi-quality videos from the beta disc from Segagaga Domain.
Also, OKeijiDragon found some more beta videos from an episode of Sega Video Magazine and it features the initial unveiling of NiGHTS at the Japanese Tokyo Prince Hotel way back in March 26, 1996; followed by an old, rare interview with Yuji Naka, Naoto Oshima, and Takashi Iizuka:
Included in the segment are footage of the game’s early stages of development, including a different stage layout for Spring Valley. Many of the sound effects heard are also very different, and even the music sounds unrefined! Such elements are highlighted in the video though annotations. There has been no translation for the interview, yet.
Thanks to Yakumo for the contribution!