The ".part1.rar" extension indicates this is the first segment of a larger, split archive often found in unofficial distribution sources or large manual downloads for the software's sample content.
Composers looking for better orchestral playback within Sibelius frequently recommend these alternatives:
The library is praised for its comprehensive range of orchestral instruments, covering everything from standard strings and brass to more niche percussion. However, some experienced users find the default playback "horrible" or "weak" compared to high-end alternatives.