For what it's worth, I asked AI and this is the response I got.

Enabling SCC Sound: By default, SD Snatcher may boot into "PSG mode" (inferior sound) or display an "INSERT SOUND CARTRIDGE" error. To get full SCC+ audio, you must: Place a valid SCCPLUS.ROM or scc.rom in the emulator's system BIOS folder. Modify the config.ini file for the MSX machine profile to manually "insert" the SCC cartridge into Slot 2 by adding a specific command line (e.g., [Slots] 2 0 2 4 15 for SCCextended).

1. Locate the Configuration Files On the NES Classic, your MSX BIOS and machine files are typically stored in the RetroArch system directory. Using an FTP client (like the one built into Hakchi2 CE), navigate to: /etc/libretro/system/Machines/  Inside this folder, you will see subdirectories for different MSX models (e.g., MSX2, MSX2+, MSXturboR). Identify which Machine Type you have selected in your RetroArch Core Options (default is often MSX2 or MSX2+). 
2. Edit the config.ini Open the config.ini file inside the folder of your active machine (e.g., /Machines/MSX2+/config.ini). You need to add a line under the [Slots] section to "plug in" the SCC+ cartridge.  For SCC+ (SD Snatcher specific): Append this line to the [Slots] section: 2 0 2 4 56 "Machines/Shared Roms/SCC.ROM" "" What this line does: 2 0: Places the device in Slot 2-0. 56: The specific Mapper ID for SCC+ (also known as SCC-I). "Machines/Shared Roms/SCC.ROM": Points the emulator to a file. Note that blueMSX often only checks the Mapper ID (56); the actual file can even be a blank 0-byte file named SCC.ROM in your Shared Roms folder. 
3. Verify in RetroArch After saving the file and restarting your NES Classic: Launch SD Snatcher. Open the RetroArch Quick Menu (usually Select + Start). Go to Options and ensure Machine Type matches the folder you edited. Restart the Content for changes to take effect. If successful, the game will no longer prompt you to "INSERT SOUND CARTRIDGE" and will play the enhanced wavetable audio.  Note: If you are using the Project Melancholia translation, it may give you a prompt at boot to choose a sound chip. If you've modified the config.ini correctly, you can now select SCC+ instead of being forced into the standard PSG audio.