Earlier BIOS revisions (such as SCPH-1002) were notorious in the emulation community for having audio glitches or compatibility issues with specific titles. The "550x" series was the standard for "universal" compatibility for a long time.
European televisions in the 90s ran at 50Hz (PAL), while US/Japanese TVs ran at 60Hz (NTSC). The PS1 BIOS handles the video output timing differently. playstation scph5502 v30 europe bios scph5502bin