As missions are quite new, I can only tell from my experience:
How missions work in general
Every mission explains itself in its short description. Most of them are "win a complete match on this specific map" or "get a number of kills in this specific game mode". After you completed this mission, you get rewarded with a weapon drop.
A little bit more in detail
You do have 2 missions at start. Every mission is explained quite simple and in order to achieve it / get your reward you need to finish the map you would complete the mission - so don't leave to early.
For example: "Get 6 kills in game mode Destroy." This means you have to look for a game in this game mode. You will join a server and you likely will kill 6 people. After you have killed those guys, be sure you stay until this map is finished.
In the end screen score board, you will receive your mission reward as a weapon drop. (Since now I only got trash drops, I don't know if they cover up uncommon or rare items, too. All items seem to be out of the new added items, no old ones.)
How to get new/more missions
For this you only have to play one supported game in the finding a game screen. This covers up match making, Gungame, Destroy and so on. In the end of the game, where weapons drop, you will now have an additional chance of a mission drop.
So if you need some missions, you may play a few Gungames, as they're finished quite fast and you get the chance for a mission drop after each game.
Powerlord is correct here. Unless you host your own server which specifically plays the "Headshot" sound from UTournament, you won't be able to play a soundfile when you kill someone with a headshot.
The closest thing could be having, say HLDJ, have 1 clip with the headshot sound, and bind it to a key that would play that sound to all your teammates (or globally if alltalk is on).
Best Answer
If I'm not mistaking, you have to create a game with no bots, and just add your friends through the Steam overlay.