Whilst I haven’t tried it myself (as I have no need) I would imagine there are two applications that may make it easier without completely automating it; there’s a plasma applet called HDMI Switcher and you might also be able to use VeroMix to make a plasma widget on your KDE desktop that would allow easy switching between your normal audio output and HDMI output.
You can install them via right click desktop -> add widget -> get new widgets in the bottom right corner. Veromix at least should allow one click changing of audio outputs.