Rockboy can finally plaback sound!!!
All Gameboy roms can playback in-game sounds, though the sound itself is still "very" choppy due to lack of optimization in code.
Many thanks to HCl and XShock for their exquisite work!
For those that
have flashed here is a "Grayscale with Sound" Rockboy build:
ftp://titania.student.utwente.nl/grayrockbox.zip
(Simply replace the files into the root folder)