Sorry for too long preface/introduction ;) but it was my very _first_ contact with ZXpand module for ZX81 computer (32Kb or RAM, SD-card instant loading plus additional AY-interface in standard of classic ZX81′s ZonX AY-interface from 80s).
It is really ZX81 playing music via AY chip (well, actually its Japanese version: YM). You can get both modules from RWAP Software: http://www.rwapsoftware.co.uk/zx812.html
via ZX81 computer with AY-interface & ZXpand Module. – YouTube.
These interfaces can be purchased easily through sellmyretro.com – we have also now produced ZXpand specific keyboard membranes for the ZX81, which have the new keywords printed on them (so it makes it easier to use).
Even better – not only does the additional AY-module for the ZXpand add 3 channel sound to the ZX81, but you can now couple it with a MrX Sound Interface, to provide 6 channel Turbo AY Sound – creating some wonderful musical possibilities
See http://www.sellmyretro.com/offer/details/Brand_New_Sinclair_ZX81_ZXpand_Interface-2435