It's only set and used by a single function.

Some popular ALSA plugins claim support for a large number of channels
because they have the capability to mix that down to the actual hardware
configuration.

Instead of defaulting to 5.1 Surround sound for these plugins, let's
fall back on the much more common Stereo configuration.  Users with
actual 5.1 Surround setups can tell us about it in winecfg.

Also, send WM_NOTIFY mesages when collapsing the previous selection and
expanding the new one.

I had to duplicate idl unfortunately, because typelib in riched20 is not
compatible with interfaces from tom.idl - method names are different,
while vtables are identical.