So I recently downloaded Rayman 2 The Great Escape from the Dreamcast section of this site, specifically this copy here. It came as a single archive containing multiple, and after extracting fully I got the full .cdi file. I downloaded the latest version of NullDC from their official website, and got this BIOS pack.

After some research I found I would need to rename a couple of the BIOS files, so I renamed "DC - BIOS.bin" and "DC - Flash.bin" to "dc_boot.bin" and "dc_flash.bin", respectively, allowing me to actually load the emulator. Next, I did File > Normal Boot and selected the Rayman .cdi file. It gives me the standard DC boot screen, has me set the clock, then drops me into the music player. Backing out of that and hitting Play gives me "Please insert game disc".

Is there maybe some crucial step I missed here, or maybe a different emulator I could be using? This is the first time I've ever done DC emulation as I own an actual Dreamcast, but just have no way of hooking it up right now.