Audio is *surprisingly* tricky on both platforms. NES has DPCM, which cheerfully interrupts the CPU and causes untold issues with controller reading. Gameboy has envelopes that are semi-required with phase-resets, and a wave channel which you in theory cannot write new samples to, and developer hacks to work around both. Both systems have various hardware revisions that have subtly different behavior, and a handful of games that will break on that specific model. It's fun!
It's also more involved to verify operation. Graphics are easier to "measure" and compare. Sound is also much more timing-dependent, e.g. when timers are running on their own all the time. It's disconcerting when sound is slightly off in games.
