Make all mix audio tracks use `type = voice`

[BC46]

Currently all mix audio tracks which are played during the story cutscenes do not specify a type. As a result, a default type is assumed, causing the audio to not be audible through the dialogue channel. If a player were to have the sound and ambient volume minimized, then they wouldn't be able to hear the mix audio at all. The mix audio tracks are clearly about the dialogues, so therefore it'd be better if they all use type = voice.

Another thing: I happen to know that many players prefer having the dialogue volume turned up while lowering the ambience and music volume. At the same time people complain that some cutscenes are too quiet. Making sure the mix audios play on the voice channel would solve this problem as well. Hence this may fix https://github.com/FLHDE/freelancer-hd-edition/issues/99 too.