The APPLE //c has no expandable slot and it has no support by sound card such as Mockingboard.
So, If you play some game on //c, It would be some disadvantage compare to APPLE //e with mockingboard.
Due to this lack of this capability, I tried to build mockingboard compatible sound card and completed designed.
Now, It works very well as the Mockingboard A/C(no voice chip).
It was required accurate timing control and firmware conflict evasion techniques.
APPLE //c doesn't have I/O slot and I needed to make required signals.
For this, I used CPU socket for it.

This sound card can be used on APPLE Iic 4000and 4100 all models.(Not //c plus nor Laser128)
Tested more than several days and no any issued encountered.
I prepared a AUX sound connector for external speaker.
And two small speaker provided for pre-order users.
Anyway, I'm receiving pre-orders from the Korean APPLE II enthusiast,
About a month later, I will put some on eBay for international APPLE II enthusiast,
If you have this device on your APPLE //c, Your APPLE//c would be almost complete sets.
If you have any concerns or question, please let me know.
Before to finalize it, I will try to apply your idea for perfect one.
Thanks;
Ian Kim
** ADDED **
Present sound card has semi-fixed sound volume so, I'm enhancing it to be able to control as followed APPLE//c Volume dial (It is located on left side of APPLE //c).
It would be controlled sound volume by APPLE //c itself without other device or connector.