The Street Electronics ECHO+ sound card complemented the popular ECHO
speech card with two AY8913 providing similar sound capabilities to
those offered by the Mockingboard.
A very clever design allowed it to use only one 6522 instead of two but
also made it incompatible with the few titles to ever support
Mockingboard sound, making it even less popular.
This routine considers the slight differences between both making it
possible to play music on this rare card.
A slot number different that slot 4 can be used for the Mockingboard.
A separate simple player introduced to include the minimum amount of
code required for the routine to work.
