Note that due to the NES audio running at 1.78MHz, and Game Boy audio at 4MHz stereo, a more sophisticated audio resampler was needed: Ryphecha (Mednafen author) has graciously written a first-rate resampler: it is a band-limited Kaiser-windowed polyphase sinc resampler. It is combined with two highpass filters to remove DC bias. The filter itself is SSE optimized, but even still, approximately 50% of CPU usage for NES/GB emulation goes to the audio filtering alone. However, you now have the best sound possible for NES and Game Boy emulation as a result.