excerpt from the bottom of page 20 and the top of page 22 of "The Apple
][ REFERENCE MANUAL"...
A program can "reference" the address of the special location for the speaker by performing a "read" or "write" operation to that address. The data which are read or written are irrevelant, as it is the _address_ which throws the switch. Note that a "write" operation on the Apple's 6502 microprocessor actually performs a "read" before the "write", so that if you use a "write" operation to flip any soft switch, you will actually throw that switch _twice_. For toggle-type soft switches, such as the speaker switch, this means that a "write" operation to the special controlling the switch will leave the switch in the same state it was in before the operation was performed.
Performing a "write" operation on the odd addresses between $C080.$C08F
should have the affect of throwing the switch twice, thus enabling the
soft switch. But, it sounds like this won't work with the 65C02. I
think a better way to do this is two "read" operations.
http://www.6502.org/documents/datasheets/