A couple of things.
1. You didn't mention what kind of hardware you are using. If the HDMI is nvidia based, you'll need to make sure you are using the nvidia-proprietary drivers in whatever OS you are using or the audio will not work.
2. If that isn't an issue, you likely just need to make your HDMI port the default sound card. If you are using pulseaudio (which a lot of OS's do), then you should be able to simply switch to the HDMI profile in the pulseaudio control app (pavucontrol IIRC). If you are not using pulseaudio, you'll need to specify the HDMI as the default sound card through an .asoundrc file. Doing that can sometimes be tricky. On my machines that I use HDMI, I install pulseaudio if its not installed already.
The KDE environment tends to automate a lot of things that the LXDE environment doesn't, KDE's aim being a complete user experience and LXDE being a little stripped down and requiring fewer system resources.