When you inactivate the function to get sunds when your contacts log in and write to you, you can reactivate the log in-sound for your prefered contacts inside Plus!. However, when you inactivate the log in-sounds, you also inactivate the sound that you get when someone writes to you, and I don't know of any way to reactivate that sound inside Plus!. Is there any way to get around this problem, or do I have to wait for the next version and hope that they've done something about it by then?

You can go into Control Panel > Sounds and remove the MSN sign-in sound, leaving sounds enabled in Messenger.

Thanks