First, I download the mp3 audio files, then use some audio editting apps like Audacity, resample the SFX to 8000 Hz, filter in 100 - 5834 Hz range, crank the volume up to 8 - 9 dB, and finally export the sound as unsigned 8-bit wav file. That's how you convert SFX to 8-bit.
Notice: GB Studio's built-in emulator still make buzz sound so you have to export the game as ROM and play on another emulator.