After reading some more I found the following two interesting threads regarding this topic:
Does anyone know how I can rewrite those pythong scripts to only switch on the tv when video is played? And where do I have to store such a python script? I was thinking the following script would suffice:
import xbmc,xbmcgui
if xmbc.getCondVisibility('Player.HasVideo'):
print '===TRUE==='
xbmc.executebuiltin('CECActivateSource')