The PC Speaker driver's ->probe() routine doesn't even get called in the
64-bit kernels. The reason for that is that the arch code apparently has
to explictly add a "pcspkr" platform device in order for the driver core to
call the ->probe() routine. arch/i386/kernel/setup.c unconditionally adds
a "pcspkr" device, but the x86_64 kernel has no code at all related to the
PC Speaker.
The patch below copies the relevant code from i386 to x86_64, which makes
the PC Speaker work for me on x86_64.