Is it possible to play a soundfile when you open a conversation?

I would have to say no, though I am not sure. I think the music would have to be in the actual characters VO sound file and even then it probably would not sound flawless as it does when you are walking around in an area.

Sorry I couldn't be of more help.

To play music at the start of a conversation, put the name of the music file in the 'Ambient Track' field of the root node of the .dlg.

The regular background music will not stop automatically, though. It has to be turned off with scripts. In TSL, the script 'a_bg_music can handle this. For K1, you need to write your own. This script will stop the background music and should be run at the beginning of the dialog:
void main() {
MusicBackgroundStop(GetArea(OBJECT_SELF));
}
At the end of the dialog, you should restart the background music again:
void main() {
DelayCommand(0.1, MusicBackgroundPlay(GetArea(OBJECT_SELF)));
}

Well, there is a way. Cause yesterday I reached the temple in Dantooine and music continued playing during the whole conversation.