rework the audio key handling to allocate a message to send to the

rework the audio key handling to allocate a message to send to the
task, rather than abusing task arguments to carry long values around.

this is unreliable, but so was the previous mechanism, just in a
different way. if you're so low on memory that you cant change the
volume, you probably have other more important problems going on
too.

tested by jim smith