On Serdashop you can found audio and video cards to be used mostly on PC. For example there are two cards which are connected to the parallel port of a PC, especially those which can't use ISA/PCI sound cards such as old portables :
Pulkomandy has created a card for Amstrad CPC(+) : Willy which lets you use an OPL3LPT or S2P on it. He wrote two programs to use the OPL3LPT which you can found on his page :
Check the CPCWiki thread for more informations and short videos of the Willy + OPL3LPT being used.
By the way, there are other ways to use MIDI on an Amstrad CPC, but it will be for another day (hint, use serial, usifac, LambdaSpeak...).