Hi guys , can you point some document / idea about controlling system volume with an api on kubuntu with plasma ?
I will need to increase / decrease volumen and also mute if possible.
Regards,
Leandro.

I would start to check Phonon API ...

EDIT:
Plasma has its own applet for audio volume management. It uses PulseAudio for controlling volume and may expose some functions over DBus.

EDIT2:
I also found a blog article about volume control in KDE Connect.

This blog is a great ideaVolume Control in KDE Connect I also experienced the same problem and found the solution there. Thank you for all the great work KDE connect + plasma browser integration is a match made in heaven. If you receive a call, YouTube is automatically paused. The one thing I would like in the remote is if the area with album art could also serve as a virtual trackpad. Whenever I am using a remote, there are some things I need a mouse for, so I have to switch between the two devices. It would be useful and functional if the album area doubled as a mouse touchpad. Thank you again for your great work.

You're looking for libpulse: https://freedesktop.org/software/pulseaudio/doxygen/

There's also a Qt wrapper around it available: https://api.kde.org/pulseaudio-qt/html/index.html